BACKGROUND OF THE INVENTION The present invention relates to a rolling control apparatus, a rolling control method and a rolling control program, and more specifically the present invention relates to selection of a final control element and feedback of a rolling mill having plural final control 5 elements and feedbacks. In the rolling mill using a tension reel for unwinding and winding of a material to be rolled, the tension reel is actuated by torque-constant control (current-constant control). A problem in the case of controlling the tension reel under torque-constant control includes that, when tension at the entry-side and the exit-side of the rolling mill fluctuates, fluctuation of 10 tension reel speed is generated to suppress it, and fluctuation of exit-side plate thickness is generated due to change of plate speed at the entry-side of the rolling mill. As a countermeasure for this, there has been carried out to allow tension fluctuation within a certain range, in order to suppress fluctuation of exit-side plate thickness, by actuating the tension reel under speed-constant control, in tension control using tension reel speed as a final control 15 element (for example, refer to JP-A-20 10-240662). In addition, in a tandem rolling mill, in the case where influence coefficient of the rolling mill changed largely by an operating condition, there has been carried out to alter a control final control element for the control state amount, as appropriate (for example, refer to JP-A-2012-176428). In the tandem rolling mill, usually, there has been carried out tension 20 control between the stands using screw-down of the latter part stand as a control final control element, and exit-side plate thickness control using thc front part stand speed as a control final co~ltroel lement. On the contrary, in the invention disclosed in JP-A-2012-176428, acquiring of the ~naximume ffect of plate thickness control and tension control becomes possible, by executing exit-side plate thickness control using screw-down of the latter part stand as a coiltrol 25 final control element, and tension co~ltroul sing front part stand speed as a control final control element, in response to a rolling state. Actuation of the unwinding side tension reel and the winding side reel by torqueconstant control (current-constant control) becoines variable factor of speed at the entry-side of the rolling mill, and speed at the exit-side of thc rolling mill, whicl~g enerate lluctuation of plate 30 thickness at the exit-side of the rolling mill. 'I'his is because, in the case of executing torqueconstant control, tension reel speed changcs by tensioll at thc elltry-side oftlle rolli~lgm ill or tension at the exit-side of the rolling mill, so as to keep torque of the tension reel constant. As a result, fluctuation of plate thickness at the exit-side is generated by the mass flow-constant rule. The most impo~-talltp oint for the material to be rolled, which is produced by the rolling mill, is precision of plate thickness at the exit-side of the rolling mill, and tension at the 5 entry-side and the exit-side of the rolling mill is impostant for stability of operation, however, its fluctuation in some degree is not a problem in view of rolling operation, as long as it is for keeping product plate thickness. Based on this way of thinking, in the invention disclosed in JP-A-2010-240662, as for deviation from a set tension value in a range set in advance, priority is given in making tension real speed constant, and fluctuation of tension real speed is suppressed 10 by not correcting the tension deviation, and the tension real is actuated under speed-constant control. In this case, it is no problem as long as the tension deviation falls within a range set in advance, however, such a case may be generated where it is over the range set in advance, depending on a rolling state or a base material condition. In that case, tension real speed results 15 in to be altered, which results in change of speed at the entry-side of the rolling mill, and generate fluctuation of exit-side plate thickness. In addition, there may also be the case where influence coefficient of the rolling mill changes depending on a rolling state, and tension control using tension reel speed as an final control element, and exit-side plate thickness control using roll gap of the rolling mill as an final 20 control element, beco~neu l~stable. In such a case, current exit-side plate thickness control using roll gap as a control final control element, and tension speed control in the case of actuating the tension reel under speed-constant control, or tension torque-constant colltrol in the case of actuating the tension reel under torque-constant control, are difficult to control stably, resulting in generation of fluctuation of plate thickness at the exit-side of the rolling mill. 25 In particular, the case where rolling speed is fast and plate thickness is thin decreases effect of plate thickness control by roll gap. Accordingly, in high speed rolling, plate thickness control by adjusting speed of the entry-side tension reel may be used in some cases. In this case, the entry-side tension control is performed by operation of roll gap, and the exit-side tension control is executed by operation of speed or torque of the exit-side TR. On the contrary, 30 in the case whcrc DCR (Double Cold Rolling) rolling for executing continuous rolling by two stands, it is necessary to col~trol3 kinds of tensions: tension of the entry-side of the rolling mill stand at the front part side, tension of the exit-side of the rolling mill stand at the latter part side, and tension between the stands between the front part stand and the latter part stand. Tension of the exit-side of the rolli~lgI nill stand at the latter part side can be controlled by a tension control apparatus at the exit-side, such as a tension reel or a bridle roll at the exit-side. Tension between the stands is tension between the fiont part stand and the latter part stand, and is in a state cut off from a te~lsionr eel at the unwi~ldi~slgid e for supplying the material to be rolled to the stand, or a tension reel at the winding side for winding the rolled 5 material to be rolled. Accordingly, in order to adjust tension between the stands, at least one of the front part stand and the latter part stand should be adjusted. In this case, it is a general way of thinking to change speed of the front part stand, using speed of the latter part stand, which is the last stand in continuous rolling, as a standard. In this case, in response to speed adjustment of the front part stand, control called "successive" is 10 executed, where control value is input also to a tension control apparatus of a tension reel or a bridle roll or the like at the entry-side of the front part stand, so that entry-side tension of the front part stand does not fluctuate. By such control, there may be the case where entry-side tension control, tension control between the stands, and exit-side plate thickness control interfere with each other to cause disturbance of exit-side plate thickness. 15 It should be noted that problems as described above may similarly become a problem, not limited to a tension reel, but also at such a configuration generating tension to the material to be rolled, at the entry-side or the exit-side of the rolling mill. Other example of a configuration generating tension to the material to be rolled at the entry-side or the exit-side of the rolling mill includes a bridle roll, a pinch roll or the like. 20 SUMMARY OF THE INVENTION A proble~nto be solved in the present invention is to suppress vibration of plate thickness at the exit-side of the rolling mill, by suitably controlling at least two rolling, and in the case of cxccuting continuous rolling at the stand, by suitably coiltrolling entry-side tension, intermediate tension and exit-side tension of the rolliilg mill. 25 One aspect of the present invention is a rolling control apparatus fbr controlling a rolling mill which continuously rolls a material to be rolled by means of two roll pairs, comprising: a roll gap control unit for controlling gap between the rolls in the roll pairs, based on tension of the material to be rolled, which is inserted to the roll pair for rolling by the roll pairs, or the material to be rolled, which is sent out from the roll pair after being rolled; a speed control 30 unit for controlling conveying speed of the material to be rolled, which is inserted to the roll pair for rolling by the roll pairs, or the material to be rolled, which is sent out from the roll pair after being rolled, based on plate thickness of the rolled inaterial to be rolled; and a tension control unit between the stands for controlling tension of the material to be rolled between the two roll pairs, by adjusting roll speed of the roll pair arranged at the latter part side in a conveying direction of the material to be rolled. In addition, other aspect of the present invention is a method for rolling control for co~ltrollinga rolling mill which continuously rolls a material to be rolled by means of two roll 5 pairs, wherein gap between the rolls in the roll pairs is controlled, based on tension of the material to be rolled, which is insei-ted to the roll pair for rolling by the roll pairs, or the material to be rolled, which is sent out from the roll pair after being rolled; conveying speed of the material to be rolled, which is inserted to the roll pair for rolling by the roll pairs, or the material to be rolled, which is sent out from the roll pair after being rolled, based on plate thickness of the 10 rolled material to be rolled; and tension of the material to be rolled between the two roll pairs is controlled, by adjusting roll speed of the roll pair arranged at the latter part side in a coilveying direction of the material to be rolled. In addition, other aspect of the present invention is a rolling control program for controlling a rolling mill which continuously rolls a material to be rolled by means of two roll 15 pairs, wherein the following steps are made to execute by information processing equipment: a step for controlling gap between the rolls in the roll pairs, based on tension of the material to be rolled, which is inserted to the roll pair for rolling by the roll pairs, or the material to be rolled, which is sent out from the roll pair after being rolled; a step for controlling conveying speed of the material to be rolled, which is inserted to the roll pair for rolling by the roll pairs, or the 20 material to be rolled, which is sent out from the roll pair after being rolled, based on plate thickness of the rolled material to be rolled; and a step for controlling tension of the material to be rolled between the two roll pairs, by adjusting roll speed of the roll pair arranged at the latter part side in a conveying dircction of thc material to be rolled. According to the present invention, vibration of plate thickness at the exit-side of 25 the rolling mill can be suppressed, by suitably controlling at least two rolling, and in the case of executing continuous rolling at the stand, by suitably controlling entry-side tension, intermediate tension and exit-sidc tension of the rolling mill. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a drawing showing the whole configuration of a rolling mill and a rolling 30 control apparatus, relevant to an embodiment of the present invention. Fig. 2 is a drawing showing rolling phenomenon of a #1 stand rolling mill, relevai~to an embodiment of the present invention. Fig. 3 is a drawing showing relationship among an entry-side teilsion suppression system, tension reel control and roll gap control, relevant to an embodiment of the present invention. Fig. 4 is a drawing showing an example of a time series of each parameter beginning with roll gap changing amount. 5 Fig. 5 is a drawing showing one aspect of relationship between a control final control element and control state amount of a single-stand rolling mill. Fig. 6 is a drawing schematically showing one aspect of cross response of a single-stand rolling mill. Fig. 7 is a drawing showing a relationship example of a control final control 10 element and control state amount of a single-stand rolling mill. Fig. 8 is a drawing showing relationship of a final control element and control state amount, in consideration of a cross term. Fig. 9 is a drawing showing fundamental parameters relevant to rolling phenomenon. 15 Fig. 10 is a drawing showing parameters relevant to DCR rolling. Fig. 11 is a drawing showing parameters relevant to DCR rolling. Fig. 12 is a drawing showing relationship among a neutral point, forward slip and backward slip, relevant to rolling. Fig. 13 is a drawing showing characteristics of tension control in response to a 20 control mode. Fig. 14 is a drawing showing function of a control method selection apparatus. Fig. 15 is a drawing showing one aspect of motion of an optimum control method determination apparatus. Fig. 16 is a drawing showing an example of an optimum control method 25 determination procedure. Fig. 17 is a drawing showing motion outline of a coiltrol output selection apparatus. Fig. 18 is a drawing showing motion outline of an entry-side TR speed command apparatus. 3 0 Fig. 19 is a drawing showing motion outline of a tension control apparatus between the stands. Fig. 20 is a drawing showing motion outline of a speed standard selection apparatus. Fig. 2 1 is a drawing showing a hardware configuration of infosn~ationp rocessing equipment relevant to the present embodiment. Fig. 22 is a drawing showing internal function of screw-down plate thickness control, speed plate thickness control, speed tension control, and screw-down tension control, relevant to an embodiment of the present invention. 5 DESCRIPTION OF THE EMBODIMENTS Explanation will be given in detail below on the present invention with an example of the most fundamental DCR (Double Cold Rolling) rolling mill, which is the rolling mill using a tension reel for unwinding and winding of the material to be rolled, and installed with an entry-side TR (tension reel) for unwinding of the material to be rolled at the entry-side of 10 the two rolling mill stands for executing continuous rolling, and an exit-side TR for winding the material to be rolled at the exit-side. Fig. 1 is a block diagram showing a control configuration of a rolling system relevant to the present embodiment. As shown in Fig. 1, the rolling system relevant to the present embodiment is a 2-stand continuous rolling mill configured by two rolling mills of a #1 15 stand rolling mill 1, and a #2 stand rolling mill 2, and is installed with an entry-side TR 3 at the entry-side of the rolling mill, and an exit-side TR 4 at the exit-side of the rolling mill. Each rolling mill is a roll pair for rolling by sandwiching the material to be rolled from up and down sides, where each roll speed is controlled by mill speed control apparatuses 12 and 22 for controlling roll speed, and roll gap is controlled by RG (Roll Gap) control apparatuses 11 and 21, 20 for controlling the upper and lower roll gap. I11 rolling operation, speed command value output from a rolling spced setting apparatus 73 is input to the mill speed control apparatuses 12 and 22 via a speed standard selection apparatus 80, respectively. In this way, the mill speed control apparatuses 12 and 22 control speed of the #1 stand rolling mill 1 and the #2 stand rolling mill 2, that is, circumferential 25 speed of the upper and lower working rolls, based on each input speed command value. Here, speed command value output by the rolling speed setting apparatus 73 may be added with necessary correction by the speed standard selection apparatus 80, however, in principle, it is used as setting value of mill speed of the #2 stand rolling mill 2. On the contrary, because ratio of speed of the #I stand rolling mill 1 relative to speed of the #2 stand rolling mill 30 2 is determined from product specifications of the nlaterial to be rolled, in the speed standard selection apparatus 80, speed set value of the #1 stand rolling mill 1 is generated based on information of speed ratio set at a spced ratio setting apparatus 76, and speed set value input from the rolling spced setting apparatus 73. The speed set value of the #I stand rolling mill 1 is input to the mill speed control apparatus 12 as well as to an entry-side TR speed command apparatus 65 via an entry-side speed setting apparatus 77. In addition, the entry-side TR 3 and the exit-side TR 4 are controlled rotation thereof by an entry-side TR control apparatus 32 and an exit-side TR control apparatus 42, 5 respectively. The entry-side TR control apparatus 32 and the exit-side TR control apparatus 42 give predetermined tension to the material to be rolled, respectively, by each motor torque given to the entry-side TR 3 and the exit-side TR 4, by controlling current of the motor for rotating the tension reel, so as to attain input current command value. An entry-side and exit-side tension current converters 15 and 16 compute current set value (motor torque set value), so as to attain 10 tension set value, based on a model of a TR (tension reel) mechanical system and a TR (tension reel) control apparatus. However, because this control model includes error, by using actual tension measured by an entry-side tension meter 5 1 installed at the entry-side of the #1 stand rolling mill 1, and an exit-side tension meter 53 installed at the exit-side of the #2 stand rolling mill 2, 15 tension set value is added with correction by an entry-side tension control apparatus 13 and an exit-side tension control 14, and given to the entry-side tension current converter 15 and exit-side tension current converter 16. In this way, the entry-side tension current converter 15 and exitside tension current converter 16 alter current commaild value to be set to the entry-side TR control apparatus 32 and the exit-side TR control apparatus 42. 2 0 An entry-side tension setting apparatus 71 and exit-side tension setting apparatus 72 set the torque (current) of the entry-side TR 3 and the exit-side TR 4, necessary to get tension of the material to be rolled, which is determined from product specificatiolls of the material to be rolled, to the entry-side tension current converter 15 and thc exit-side tension current converter 16, respectively. In addition, roll gap of the #1 stand rolling mill 1 and the #2 stand rolling mill 25 2 was set to a #1 stand screw-down position setting apparatus 74 and a #2 stand screw-down position setting apparatus 75, respectively, based on product specifications of the material to be rolled, and given to the RG control apparatuses 11 and 21. As fundamental control applied to the DCR rolling mill, control by the entry-side tension control apparatus 13, the exit-side tension control apparatus 14, a tension control 30 between the stands apparatus 67 and a screw-down plate thickness control apparatus 61 is a major one. The entry-side tension control apparatus 13 adjusts tension set value to be given to the entry-side tension current converter 15, based on the entry-side tension of the #1 stand rolling mill 1 (hereafter abbreviated as the entry-side tension) measured by the entry-side tension meter 5 1. The exit-side tension control apparatus 14 ad-justs tension set value to be given to the exit- 9 - side tension current converter 16, based on the exit-side tension of the #2 stand rolling mill (hereafter abbreviated as the exit-side tension) measured by the exit-side tension meter 53. The tension control between the stands apparatus 67 adjusts speed command value to be given to the mill speed control apparatuses 12 and 22, and the entry-side TR speed 5 command apparatus 65, based on tension between the #1 stand rolling ~nill1 and the #2 stand rolling mill 2 (hereafter abbreviated as the tension between the stands) measured by the tension meter between the stands 52. Among the above 3 tension controls, control by the tension control between the stands apparatus 67 is managed by a control method selection apparatus 70. This point is one of the gist relevant to the present embodiment. Detail will be described later. 10 In addition, because plate thickness of the material to be rolled is important in view of product quality, plate thickness control is executed. The screw-down plate thickness control apparatus 61 adjusts command value of roll gap of the #1 stand rolling mill given to the RG control apparatus 11, based on exit-side plate thickness of the #2 stand rolling mill 2 (hereafter abbreviated as the exit-side plate thickness) measured by the exit-side plate thickness 15 meter 54. It should be noted that processing of the screw-down plate thickness control apparatus 61 is also managed by the control method selection apparatus 70. This point is one of the gist relevant to the present embodiment. Detail will be described later. In the DCK rolling mill, plate thickness of the material to be rolled having a plate thickness of a base material of about 0.2 mm to 0.4 mm, is made thin under a rolling reduction of 20 about 20% to 35% by the #I stand rolling mill 1, and in the #2 stand rolling mill 2, screw-down is executed in a degree so that plate thickness does not change for temper rolling. Accordingly, as rolling condition in the #1 stand rolliiig mill 1, there is generated the case where exit-side plate thickness becomes thin, and rolling speed becomes high speed. Fig. 2 is a drawing showing rolling phenomenon of a #1 stand rolling mill of the 25 DCR rolling mill. Rolling phenomenon of the #I stand rolling mill of the DCR rolling mill is similar to rolling phenomenon in a single-stand rolling mill. Here, a control configuration of the entry-side tension TL, as torque-constant control of the entry-side TK 3, and plate thickness control of an exit-side plate thickness h using roll gap of the #1 stand rolling mill 1 as an final control element, as shown in Fig. 2, has a problem that exit-side plate thickness and entry-side 30 tension cannot be controlled stably. Explanation will be given below on such a problem. As a f~~ndamentraull e in the rolling mill, there is a mass flow-constant rule. This is expressed by the following cxpression (1), using an entry-side platc thickness 11, the exitside plate thickness 11, an entry-side plate spced V,, and an exit-side plate speed V,, in the premise that the material to be rolled is in a continuous statc at the entry-side of the rolling mill stand and the exit-side of the rolling mill stand: H.V, = h.Vd (1) From expression (1) of the mass flow-constant rulc, in the case where the entryside plate thickness H is constant, it means that fluctuation of the entry-side plate speed V, 5 fluctuates the exit-side plate thickness h. As described above, in the DCR rolling mill, because main rolling is executed by the #1 stand rolling mill, the entry-side plate speed V, in the above expression (1) becomes speed of the entry-side TR 3. Speed of the entry-side TR 3 changes so that tension torque coincides with motor torque, however, because this change is executed by inertia of the entry-side TR 3 and the above-described control, there is no control means for 10 suppressing change of the entry-side plate speed V,. When adjustment of roll gap by the RG control apparatus 11 is executed, in order to suppress change of the exit-side plate thickness h caused by change of this entry-side plate speed V,, forward slip and backward slip change by change of rolling reduction, resulting in change of the entry-side plate speed V, and the exit-side plate speed Vd, which still more 15 generates change of the entry-side tension Tb. In order to suppress this, speed of the entry-side TR 3 fluctuates, as described above, and fluctuation of exit-side plate thickness is generated still more by this fluctuation. In this way, there may be the case where the entry-side tension suppression system executed by the entry-side TR 3 has large time constant depending on rolling condition, causing fluctuation of exit-side plate thickness having large waviness. Causes 20 generating such waviness are called cross terms, because influence by control gives influence each other. The entry-side tension Tb of the rolli~lgm ill stand is suppressed also by rolling phenomenon. When the entry-side tension Tb fluctuates, rolling load of the rolling mill changes, and by accompanying change of forward slip and backward slip, the entry-side plate 25 speed V, and the exit-side plate speed Vd fluctuate. The entry-side tension Tb fluctuates also caused by this entry-side tension rolling phenomenon system. Response of the entry-side tension rolling phenomenon system is very fast as co~nparedw ith the above-described entry-side tension suppression system, and the entry-side tension rolling phenomenon system and the entryside tension suppression system can be expressed as shown in Fig. 3. 30 From Fig. 3, it is understood that a roll gap alteration amount AS of the rolling mill stand is expressed as deviation ATb of entry-side tension in the same phase, and entry-side TR speed changes in a state that it is integrated at the entry-side TR 3. Accordingly, the deviation AT,, of the roll gap alteration amount AS and entry-side tension, change of entry-side 'I'R speed, and change of exit-side plate thickness become such a relation as shown in Fig. 4. - 11 - Fig. 4 is a drawing showing relationship among the roll gap alteration amount AS, the entry-side tension TI,, the entry-side TR speed, and the exit-side plate thickness h. As shown in Fig. 4, when the roll gap alteration amount AS changes, entry-side speed of the rolling mill stand changes, and the entry-side tension Tb changes. Accompanying 5 with change of the entry-side tension Tb, entry-side TR speed changes by motion of inertia of the entry-side TR 3, because the entry-side TR 3 is executing torque-constant control. When entryside TR 3 speed fluctuates, fluctuation of exit-side plate thickness is generated by the mass flowconstant rule shown in the above expression (1). When fluctuation of exit-side plate thickness is generated, the screw-down plate thickness control apparatus 61 operates the roll gap change 10 amount AS so as to make exit-side plate thickness constant. Continuation of these series of motions makes vibration of exit-side plate thickness, as shown in Fig. 4. It should be noted that, because an exit-side plate thickness meter 17 is practically installed at a place apart from the rolling mill 1, there is present time delay up to detection of exit-side plate thickness which an exit-side plate thickness control apparatus 18 uses, however, it 15 can be neglected in the case where the delay time is sufficiently short relative to vibration frequency of exit-side plate thickness. In the rolling mill, there are present two control final control elements: roll gap and roll speed, and two control state amounts: exit-side plate thickness of the rolling mill and entry-side (or exit-side) tension of the rolling mill. In the case where two control final control 20 elements were operated, it influences on each of the two colltrol state amounts, which then alters the control state amounts. Fig. 5 is a drawing showing relationship between such a control final control element and control state amount, in the case of one rolling mill stand. Rolling phenomenon of one rolling mill stand becomes as shown in Fig. 2, and Fig. 5 is the one described this conceptually. 25 I11 the casc of one rolling mill stand, the control final control elements are the roll gap alteration amount AS, and speed of the exit-side TR or the entry-side TR of the front part stand (hereafter it is represented as "entry-side TR speed"). The entry-side plate speed V, is determined by this entry-side 'I'R speed. In addition, the control state amounts are the exit-side plate thickness h and the entry-side tension Tb of the rolling mill. In the case where the roll gap 30 alteration amount AS was altered, change of the exit-side plate thickness h by a (roll gap + exit-side plate thickness) influence coefficient 503, and change of the entry-side tension Tb by a (roll gap + entry-side tension) influence coefficient 501 are gcnerated. In addition, in the casc where V, changed caused by changing of TR speed at the exit-side or the entry-side of the fiont part stand, change of ihe entry-side te~ision caused by an (entry-side TR speed + - 12 - entry-side tension) influence coefficient 502, and change of the exit-side plate thickness h caused by an (entry-side TR speed -+ exit-side plate thickness) influence coefficient 504 are generated. As described above, the rolling mill exit-side plate thickness h can be controlled by alteration of roll gap, based on measurement result of the exit-side plate thickness meter 54. 5 In addition, the entry-side tension Tb is controlled by change of entry-side TR speed, based on difference between noto or torque and entry-side tension torque. On the contrary, in consideration of relationship as shown in Fig. 5, when roll gap is altered, change of not only the exit-side plate thickness h but also the entry-side tension Tb occurs, while when entry-side TR speed is altered, change of not only the entry-side tension Tb but also the exit-side plate thickness 10 h occurs. This change is not-intended unnecessary change. In the relationship shown in Fig. 5, in the case where the (roll gap + exit-side plate thickness) influence coefficient 503 and the (entry-side TR speed -+ entry-side tension) influence coefficient 502 are sufficiently large as compared with the (roll gap -+ entry-side tension) influence coefficient 501 and the (entry-side TR speed -+ exit-side plate thickness) 15 influence coefficient 504, this control configuration has no problem, because influence of the above-described not-intended unnecessary change is small. On the contrary, in the case where the (roll gap -+ exit-side plate thickness) influence coefficient 503 and the (entry-side TR speed + entry-side tension) influence coefficient 502 become smaller as compared with the (roll gap -+ entry-side tension) influence 20 coefficient 501 and the (entry-side TR speed -+ exit-side plate thickness) influence coefficient 504, a problern is generated that stable control is not executed, because influence of the abovedescribed not-intended unnecessary change becomes large. When such a state appears, because the exit-side plate thickness h is controlled based on measurement result of the exit-side plate thickness meter 54, eve11 by operation of roll 25 gap of the #1 rolling mill stand 1, the entry-side tension Tb fluctuates largely, and in order to control this, change of entry-side TR speed occurs caused by difference between motor torque and entry-side tension torque. As a result, the exit-side plate thickness h fluctuates largely. When the exit-side plate thickness h changes, roll gap operation by the screw-down plate thickness control apparatus 6 1 is executed based on measurement result of the exit-side plate 30 thickness meter 54, resulting in generation of a state that the exit-sidc plate thickness h, the entry-side tension T,,, the entry-side plate speed V,, and the roll gap S vibrate under the same fi-ecluency. As for rolli~~pghe nomenon as explained in Fig. 3, s~icha rolling phenomenon system is shown in Fig. 6, where an entry-side tension suppression system by the entry-side TR 3 is removed off, the entry-side plate speed V, by operation of entry-side TR speed, and the roll gap alteration amount AS are used as control final colltrol elements, and the exit-side plate thickness h and the entry-side tension Tb are used as control state amounts. In Fig. 6, the entry- 5 side tension rolling phenomenon system are put together to give an entry-side tension influence coefficient 101. From Fig. 6, 11 1, 112, 1 13 and 1 14 of Fig. 7 are obtained, as those corresponding to the influence coefficients 50 1, 502, 503 and 504 in Fig. 5. According to Fig. 7, it is understood that, when the exit-side plate thickness h is thin, and the entry-side plate speed V, is fast, the (entry-side TR speed + exit-side plate 10 thickness) influence coefficient 11 4 and the (entry-side TR speed + entry-side tension) influence coefficient 112 become small. In addition, a first-order lag time constant Tr included in the entry-side tension influence coefficient 101 becomes small. Accordingly, the (roll gap + exit-side plate thickness) influence coefficient 11 3 becomes small. In addition, the (roll gap -+ entry-side tension) influence coefficient 1 11 becomes fast in response. 15 That is, when the exit-side plate thickness 11 is thin and the entry-side plate speed V, is fast, in operation of the roll gap AS, the exit-side plate thickness h of the rolling mill becomes difficult to change, and the entry-side tension Tb becomes easy to change. That is, the (roll gap + entry-side tension) influence coefficient 11 1 becomes larger than the (roll gap + exit-side plate thickness) influence coefficient 1 13. In addition, in operation of the entry-side 20 plate speed V,, the entry-side tension 'rh and the exit-side plate thickness h become difficult to change similarly. The entry-side tension Th includes a rolling phellolnenon term K,,. The Kb also changes in response to rolling speed and exit-side plate thickness, and when the Kb becomes large, the (entry-side TR speed + entry-side tension) influence coefficient 112 becomes smaller 25 as compared with the (entry-side I'R speed + exit-side plate thickness) influence coefficient 114. As described above, it is understood that there is present the case where, when the exit-side plate thickness h becomes thin and the entry-side plate speed V, becomes fast, the (roll gap + exit-side plate thickness) influence coefficient 11 3 becomes smaller as compared with 30 the (roll gap + entry-side tension) influence coefficient 11 1, and the (entry-side TR speed -+ entry-side tension) influence coefficient 11 2 becomes smaller as compared wit11 the (entry-side TR speed + exit-side plate tl~ickness)i nfluence coefficient 114. In suc11 a case, in trying to execute suc11 control as shown in Fig. 3, that is. control of the exit-side plate thickness 11 by the - 14- roll gap alteration amount AS and control of the entry-side tension Tb by the entry-side plate speed V,, stable control becomes impossible, because influence of the cross term is large, as described above. In such a case as this, as shown in Fig. 8, by actuation of the speed plate thickness 5 control apparatus 62, which controls the exit-side plate thickness h by change of the entry-side plate speed V, by controlling the entry-side TR speed, and the screw-down tension control apparatus 64, which controls the entry-side tension Tb by roll gap alteration amount AS, the exitside plate thickness h and the entry-side tension TI, become to be controlled stably. To realize this, it is necessary to change the entry-side TR 3, which is working under conventional torque- 10 constant control (current-constant control), to working under speed-constant control. Accordingly, the entry-side TR speed command apparatus 65 is installed. Also in the case where response of the entry-side tension suppression system became deteriorated, it is necessary to work the entry-side TR 3 under speed-constant control. The entry-side tension suppression system in Fig. 3 becomes a first-order lag system of time 15 constant T,, by equivalent transformation. Here, the Tq is proportional to the entry-side plate speed V,, inversely proportional to the exit-side plate thickness h of the rolling mill, and proportiollal to the rolling phenomenon term Kb. Accordingly, when the rolling phenomenon term Kb becomes large, the time constant T, of the entry-side tension suppression system becomes large, and response of the entry-side tension suppression system becomes deteriorated. 20 In addition, in this case, because the (roll gap + entry-side tension) influence coefficient 11 1 in Fig. 5 does not become large, it is considered that stable control is possible by plate thickness control by the conventional roll gap alteration amount AS and the speed tension control 63 which operates speed of the entry-side TR 3. And, in a rolling apparatus, the material to be rolled having various material 25 qualities is rolled to various plate thicknesses, and rolling speed is also various. Accordingly, it is preferable to switch a control method in response to a rolling state, in consideration of also change of control response as described above. In the rolling control method relevant to the present embodiment, by switching the 3 kinds of control methods of the following (A) to (C), exit-side plate thickness and control of entry-side tension is stabilized. 30 (A) plate thickness control operating the roll gap, and tension control by an entry-side tension suppressio~s~ys tem of the entry-side TR working under torque-constant control (B) plate thickness control operating the roll gap, and speed tellsioll control by operating the speed of the entry-side TR working under speed-constant control, and (C) screw-down tension control operating the roll gap, and speed plate thickness control by operating the speed of the entry-side TR working under speed-constant control. In the DCR rolling mill, because maintaining of tension between the stands between the #1 stand rolling mill 1 and the #2 stand rolling mill 2 is also important, the tension control between the stands apparatus 67 is installed. 'The tension control between the stands 5 usually regards speed of the #I stand rolling mill 1 as a final control element. Fig. 9 is a drawing showing fundamental rule of rolling including the abovedescribed mass flow constant rule. As shown in Fig. 9, the mass flow constant rule shown in the above-described expression (I) is held between plate thickness and speed at the entry-side and the exit-side of the rolling mill stand. In addition, the material to be rolled becomes to 10 extend in a length direction by screw-down. Ratio of extension in this length direction is expressed by a forward slip f and a backward slip b. And, relationship of the following expressions (2) and (3) is held among this forward slip f and the backward slip b, along with the roll speed of the rolling mill stand Vr<, the entry-side plate speed Ve and the exit-side plate speed Vd of the rolling mill stand. 15 Ve = (I -b).VR (2) Vd = (1 +f).VR (3 On the other hand, between the #I stand rolling Inill 1 and the #2 stand rolling mill 2, or between the entry-side TR 3 and the #1 stand rolling mill 1, or between the #2 stand rollillg mill 2 and the exit-side TR 4, the following expression (4) is held, using the tension 20 between the stands T, a plate cross section area A, a distance between the stands L, a # 1 stand entry-side speed V,,, a #1 stand exit-side speed Vd,, as shown in Fig. 10. (a) of Fig. 11 is a drawing showing relationship of roll speed of each rolling mill stand and entrylexit-side apparatuses in normal rolling (a state that plate thickness and tension 25 are constant values). In paying attention to entry-side tension of the #1 stand rolling mill 1, it is necessary that entry-side speed of the #1 stand rolling mill 1 and speed of the entry-side TR 3 are the same speed. that is, VTTR=Vciln, order to attain constant tension. I Iere, it is assumed that speed 01t he #1 stand rolli~~mgil l 1 was altered by actuation of the tension control between the stands apparatus 67. Because control output 30 (l+AVlV) alters speed of the #1 stand rolling mill 1, entry-side tension beconles altered, when no action is taken. Accordingly, as shown in (b) of Fig. 11, such function becomes necessary that similar control output (I +AV/V) is output also to the entry-side TR to make speed of the entry- 16 - side TR 3 coincided with entry-side speed of the #1 stand rolling mill 1, so as not to generate tension fluctuation. Tlle one executing this is a successive function. As the successive function, use of known various functions is possible. Here, it is assumed that the forward slip f and the backward slip b, which 5 determine exit-side speed and entry-side speed of the rolling mill, as explained in Fig. 9, are constant. Plate thickness fluctuation caused by tension fluctuation, and thus mass flow fluctuation at the entrylexit-side of the stand of the rolling mill, in operation of speed of the rolling mill, have been prevented by utilization of successive function, assuming fluctuation of the forward slip f and the backward slip b is slight, although not constant, during rolling, and as 10 being negligible one. However, in the case where fluctuation of the forward slip and the backward slip are over and larger than the negligible level, there is the case where utilization of the successive function becomes, on the contrary, disturbance to tension or plate thickness. In particular, influence of fluctuation of the forward slip f and the backward slip b, is large, which is generated 15 by fluctuation of the neutral point in rolling, that is, the point where roll speed coincides with speed of the material to be rolled, and there is the case where the successive function interferes with roll gap control or the tension control between the stands 67, based on measurement result of the exit-side tension meter 54, along wit11 the entry-side tension control based on measurement result of thc entry-side tension meter 5 1, and becomes vibration phenomenon of plate thickness 20 or tension, similar to that explained in Fig. 4. Fig. 12 shows relatiollship between the neutral point, the forward slip and the backward slip, in rolling. Rolling is executed by passing througl~th e material to be rolled 5 between an upper working roll 101 and a lower worlcing roll 102. In that time, slip is generated between the material to be rolled 5 and the upper and lower working rolls 101 and 102, and one 25 neutral point, where roll speed coincides with speed of the material to be rolled 5, is generated at a region where the roll contacts with the material to be rolled. Speed at a contact beginning point of the work roll and the material to be rolled becomes the cntry-side speed V,. In addition, speed at a contact ending point of the work roll and the material to be rolled becomes the exit-side speed Vd. 3 0 'I'he forward slip f is the one where 1 is subtracted from ratio of the exit-side speed Vd and the neutral point speed VIi, (Vd/VIc)a, nd the backward slip b is the one where ratio of the entry-side speed V, and the neutral point speed Vp,, (V,/Vn) is subtracted from 1. In the case wl~erera tio ofthe entry-side speed V, and the exit-side speed Vd, (Ve/Vd)c l~angesf,o r example, the exit-side plate thickness 1-1 becomes thick, the entry-side speed V, becomes slow, thc exit-side speed Vd becomes fast, the successive function should be actuated. In the case wllere only the position of the neutral point changes, while the ratio of the entry-side speed V, and the exit-side speed Vd, (Ve/Vd) is kept constant, it is necessary to change the roll speed VR according to change of the position of the ~~eutrpaoli nt, by wl~ichth e 5 entry-side speed V, is kept constant, therefore speed of the entry-side TR 3 should not be altered using the successive function. For example, in Fig. 12, assuming that the position of the neutral point changed from a neutral point A to a neutral point B, the forward slip f becomes small, and the backward slip b becomes large. According to this, the exit-side speed Vd becomes small as explained in 10 Fig. 9, and the tension between the stands T becomes large as explained in Fig. 10. In addition, the entry-side speed V, also becomes small, and entry-side tension becomes small. On the contrary, in the case where the position of the neutral point changed from the neutral point B to the neutral point A, the tension between the stands T becomes small, and entry-side tension becomes large. That is, entry-side tension and tension between the stands change in a reversed 15 phase. In the case of accompanying plate thickness fluctuation, generally, the entry-side speed V, and the exit-side speed Vd change in a reversed direction, and the entry-side tension and tension between the stands change in the same phase. Accordingly, whether it is caused by change of the position of the neutral point, or fluctuation of plate thickness can be judged, based 20 on whether change of the entry-side tension and tension between the stands of the #1 stand rolling mill 1 is under the same phase or the reversed phase. Also in the case where roll speed of the #I stand rolling Inill 1 changed, the entryside tension and tension between the stands change in a reversed phase, and also in this case, it is better not to actuate the s~lccessivef unction. It is because both entry-side tension and tcnsion 25 between the stands can be controlled at the same time by correction of the roll speed VR, and on the contrary actuation of tllc successive function provides a state that entry-side tension is not corrected. (a) and (b) of Fig. 13 show characteristics of tension control in the abovedescribed three control modes. In the control method (A) and the control method (B), as show11 30 in (a) of Fig. 13, entry-side tension control is executed by controlling rotation of the entry-side 'I'R 3. Accordingly, influence of entry-side tension control is limited to the entry-side of the #I stand rolling mill. In addition, because exit-side tension control is executed to operate the exitside TR 4, influence of exit-side tension control is limited to the exit-side of the #2 stand rolling mill. In the case of executing tension control between the stands, operation of speed of the #1 stand rolling mill or speed of the #2 stand rolling mill is necessary, however, by executing "successive" to the entry-side TR 3, in the case of operating speed of the #I stand rolling mill, and to the exit-side TR 4, in the case of operating speed of the #2 stand rolling mill, influence 5 given to entry-side tension or exit-side tension can be suppressed. In this case, operation of speed of the #1 stand rolling mill is advantageous in view of control response, because of small ' ' S U C C ~ S S ~ Va~m~o' unt to the entry-side TR 3. In addition, in the DCR rolling mill, because rolling reduction is k 0 in the #2 stand rolling mill, in the case where the neutral point shifted by operating speed of #l stand 10 rolling mill, entry-side tension and tension between the stands can be controlled effectively. In that case, operation of entry-side TR speed by "successive" becomes disturbance, however, it can be suppressed by entry-side tension control. As explained in Fig. 12, when the case of fluctuation of the neutral point to the exit-side is considered, entry speed decreases and exit speed decreases. As a result, entry-side 15 tension decreases and tension between the stands increases. On the contrary, entry-side tension control lowers speed of the entry-side TR, and tension control between the stands increases speed of the #1 stand. By "successive", speed of the entry-side TR is made fast at the same time, and it interferes with entry-side tension control, however, entry-side tension increases due to increase 20 in #1 stand speed. In the case where the neutral point does not fluctuate, for example, in the case where #1 stand gap is closed, both entry-side tension and tension between the stands decrease, tension between the stands decreases #1 stand speed, and entry-side TR speed is decreased by "successive". The entry-side tension control controls entry-side tension by decreasing entry-side TR speed. 25 In the case of the control method (C), because control final control element of entry-side tension control is roll gap of the #1 stand rolling mill, as shown in b) of Fig. 13, it influences not only on entry-side tension but also on tension between the stands. Mere, as shown in Fig. 12, when the case of fluctuation of the neutral point to the exit-sidc is considered, entry speed decreases and exit speed decreases. Accordingly, entry-side tension decreases and 30 tension between the stands increases. On the contrary, entry-side tension control tries to return entry-side tension to original value by col~trollingr oll gap of the #1 stand rolling mill in an opening dircctio~(b~e cause entry-side tellsioll decreased). Because exit-side plate thickness also increases, the exit-side plate thicki~essc ontrol decreases entry-side TR speed and illcreases entry-side tension. By opening roll gap of the #1 stand rolling mill, also tension between the stands results in to increase. In the case where tension control between the stands regards #1 stand speed as a final control element, it tries to decrease tension between the stands by increasing #1 stand 5 speed. This increases entry-side tension, however, when "successive" is executed, increase entry-side tension does not, and entry-side tension control increases tension between the stands. Accordingly, it results in excessive operation of #l stand speed, causing interference with exitside plate thickness control. In this case, by stopping "successive" to the entry-side TR3, it is possible to suppress the interference. 10 However, in the case where there is no fluctuation of the neutral point, for example, roll gap is controlled so as to be closed, because both entry-side tension and tension between the stands decrease, by operation of roll gap of the #1 stand rolling mill by entry-side tension control, it is possible to suppress the tension between the stands. As tension control between the stands, fluctuation of entry-side tension is prevented by decreasing speed of the #1 15 stand rolling mill, and decreasing also speed of the entry-side TR by "successive". Accordingly, "successive" to the entry-side TR cannot be stopped unconditionally. This problem can be solved by regarding a control output edge of the tension control between the stands as #2 stand speed. In the case of the above-described state, by decreasing #2 stand speed, it is possible to decrease tcnsion between the stands, and in this case, 20 interference among exit-side plate thickness control, entry-side tension control, and tension control between the stands is not generated. In the case of operating #2 stand rolling mill speed, it is necessary to execute "successive" to exit-side TR speed, however, in the case of the DCR rolling mill, fluctuation of the neutral point can be neglected, because screw-down is +O at the #2 stand. 2 5 In the case where there is fluctuation of the neutral point in the #1 stand rolling mill, it is finally necessary to alter #1 stand speed, or altcr speed of the entry-side TR, the #2 stand rolling mill and the exit-side TR. In the case of the control method (A) and the control method (B), it is corresponded with by altering #I stand speed, and in the case of the control method (C), it is cossesponded with by altering speed of the entry-side TR, the #2 stand, and the 30 exit-side TR. As described above, in the DCR rolling mill, in the case of using the abovedescribed three kinds of control mcthods, by switching them in response to a rolling state, it is necessary to alter not only entry-side tension control, and exit-side plate thickness control, but also a control output edge of tension control between the stands. Alteration of the control output edge of tension control between the stands results in alteration of the rollillg mill stand (master stand), which becomes a speed standard in the tandem rolling mill. Based on such principle, explanation will be given on control of the rolling system relevant to the present embodiment, by returning back to Fig. 1. As described above, in 5 order to execute plate thickness control and tension control of the rolling mill stably, it is necessary to use the above-described 3 kinds of controls, by switching them in response to a rolling state. Accordingly, operation command AASAGc to roll gap is produced by the screwdown plate thickness co~ltroal pparatus 61, and operation command AAVAGct o the entry-side TR is produced by the speed plate thickness control apparatus 62, using the exit-side plate thickness 10 deviation Ah measured by the exit-side plate thickness meter 54. In addition, using deviation (deviation of entry-side tension) ATb between entryside tension actual result measured by the entry-side tension meter 5 1 and entry-side tension setting set by the entry-side tension setting apparatus 71, the speed tension control apparatus 63 produces operation command AAVATRto the entry-side TR speed, and the screw-down tension 15 control 64 produces operation command AAVAni to roll gap. In addition, in the case where the entry-side TR 3 is running under torqueconstant control, the one added with control output from the entry-side tension control 13 for operating entry-side tension set value to the entry-side tension set value by the entry-side tension setting apparatus 7 1, by deviation between entry-side tension result and entry-side tension set 20 value, is converted to current colninand to thc entry-side TR 3 by the entry-side tension current converter 15 to prepare current colnmaiid value to the entry-side TR control apparatus 32. The control method selectio~al pparatus 70 selects, in response to a rolling state, application of which control mctl~oda mong (A), (B) and (C) can most decrease fluctuation of exit-side plate thickness and fluctuation of entry-side tension, and based on the selection result, 25 outputs roll gap operation command to the roll gap control apparatus 11. In the case of operating the entry-side TR speed, it outputs speed operation command to the entry-side TR speed command apparatus 65. In the entry-side TR speed command apparatus 65, entry-side TR speed command is prepared by nleans of entry-side TR standard speed, which is output from the rolling speed setting apparatus 73 via the speed standard selection apparatus 80, and entry- 30 side TR speed alteration amount from the control method selection apparatus 70, and outputs it to the entry-side TR control apparatus 32. The entry-side TR control apparatus 32 has an operation mode for executing torque-constant control (current-constant control) in response to curre~lct ommand, and an operation mode for executing speed-constant colltrol in response to speed cornmand, and - 21 - operates them by switching then1 in response to co~nmandf rom the control method selection apparatus 70. Fig. 22 shows one example of a block diagram of the screw-down plate thickness control 61, the speed plate thickness control 62, the speed tension control 63, and the screw- 5 down tension control 64. They are one example of each control configuration, and a configuration using a method other than this is also possible. For example, in an example of Fig. 2, each control system is configured by integral control (I control), however, proportional integral control (PI control) or proportional integral derivative control (PID control) may also be configured. 10 The screw-down plate thickness control apparatus 61 relevant to the present embodiment is configured by integral control (I control), which regards, as input, an exit-side plate thickness deviation Ah=hfb-hrefw, hich is difference between the exit-side plate thickness actual result hfb, which is output from the exit-side plate thickness meter 54, and the exit-side plate thickness set value hres, which is set in rolling operation, and integrates the one obtained by 15 multiplying the adjustment gain and conversion gain from exit-side plate thickness deviation to roll gap, to input exit-side plate thickness deviation. Deviation between output after integral and the previous time value is used as control output AASAGc. In addition, the speed plate thickness control 62 is configured by integral control (I control), which regards, as input, an exit-side plate thickness deviation Ah, and integrates the 20 one obtained by multiplying the adjustment gain and conversion gain fronl exit-side plate thickness deviation to cntry-side speed, to input exit-side plate thickness deviation. Deviation between output after integral and the previous value is used as colltrol output by the following expression (5). This command value is output as speed alteration ratio to set speed. 2 5 Screw-down tension control apparatus 64 is configured by integral colltrol (I control), which regards, as input, an entry-side tension deviation ATb=Tbfbb-Tbrwcfh,i ch is difference between the entry-side tension actual result Tblbb, which is measured by the entry-side tension meter 5 1, and the entry-side tension set value Thrct, which is set in advance in rolling operation, and integrates the one obtained by multiplying the adjustment gain and conversion 30 gain from entry-side tension deviation ATl, to roll gap, to input elltry-side tension deviation ATl,. Deviation between output after integral and the previous time value is used as control output AASA1I1 . In addition, the speed tension co~ltroal pparatus 63 is configured by integral control (I control), which regards, as input, the entry-side tension deviation ATb, and integrates the one obtained by multiplying the adjustment gain and conversion gain from the entry-side tension deviation ATb to entry-side speed, to the entry-side tension deviation ATb. Deviation between output after integral and the previous value is used as control output by the following 5 expression (6). Fig. 14 shows outline of the control method selection apparatus 70. The control method selection apparatus 70 is configured by an optimum control method determination apparatus 70a and a control output selection apparatus 70b. In the optimum control method 10 determination apparatus 70a, it is determined which control method among the above-described (A), (B) and (c) is used in control, and in the control output selection apparatus 70b, it is selected which output of the screw-down plate thickness control apparatus 6 1, the speed plate thickness control apparatus 62, the speed tension control apparatus 63, and the screw-down tension control apparatus 64 is used, and control command is output to the roll gap control apparatus 1 I, the 15 entry-side TR speed command apparatus 65, the entry-side TR control apparatus 32, and the speed standard selection apparatus 80. Fig. 15 shows operation outline of the optimum control method determination apparatus 70a. Here, in the case where the above-described (roll gap -+ entry-side tension) influence coefficient 11 1 is large, tension control by screw-down, and plate thickness control by 20 reel spced should be executed using the control method (C), and in the case where tension corrcction time constant of the entry-side tension suppression systcm is large, plate thickness control by screw-down, and tension control by operation of TR speed should be executed using the control method (B). In other cases, the control method (A), which has been executed conventionally, should be selected. 2 5 Which one of the three control methods should be selected is determined as follows. Because the optimum control method is considered to change by steel kind of the ~naterialto be rolled, exit-side plate thickness and rolling speed, when steel kind and exit-side plate thickness changed, rolling speed is classified into about three stages of low speed, medium speed, and high speed, and when the relevant rolling speed was achieved in rolling, change of 30 entry-side tension and exit-side plate thickness is checked by changing roll gap in step wise. I11 this case, alteration amount of roll gape can be implemented even in rolling of a product material, as long as it is changed ~ I Ia n amount not to give large influence on product quality of - 23 - the material to be rolled. In addition, in the case of changing roll gap in step wise, the abovedescribed control method (A) should be selected in advance. It should be noted that in the present embodiment, as shown in Fig. 15, rolling speed is altered in step wise in the order of low speed, medium speed, and high speed. This is 5 because of performing for selection of any of the above-described three control methods. E-Iowever, also in the case of practically beginning rolling operation, rolling speed is increased in step wise, as shown in Fig. 15. Accordingly, operation as shown in Fig. 15 can be executed together with usual rolling operation, and can be executed without decreasing productivity. Fluctuation amount of entry-side tension and fluctuation amount of exit-side plate 10 thickness, just after alteration of roll gap in step wise, are measured to judge which of the (roll gap + entry-side tension) influence coefficient 114 and the (roll gap -+ exit-side plate thickness) influence coefficient 112 is larger. In addition, response time of the entry-side tension suppression system is judged from change of entry-side tension, in the case where roll gap was operated in step wise. 15 For example, as shown in Fig. 15, in response to rolling speed, regions of low speed, ~nediums peed, and high speed are determined. This determination method may be divided into three equal parts up to the maximum speed, or may be divided based on other suitable standard. When rolling speed entered these regions, step wise disturbance is added to roll gap. By adding step wise disturbance, entry-side tension and exit-side plate thickness 20 fluctuate. Next, as shown in Fig. 16, from actual result of entry-side tension and deviation of exit-side plate thickness, parameters dTI,, dh and T,,, are determined. These parameters can be determined by signal processing from a fluctuation state of time direction of actual value. From magnitude relation of thus determined parameters dTb, dh and Tbr, the control method (A), 25 the control method (B), and the control method (C) are selected. In selection of each of the control method (A), the control method (B), and the control method (C), as shown in Fig. 16, it is judged by comparison of a value calculated based on the above-described parameters dTb, dh and Tbr, with a predetermined threshold value. For example, in the case where thc value calculated by (dh/hrel)/(dTb/Tbreisl) e qual to or smaller than 30 selected value of the control method (C), which is the predetermined threshold value, the control method (C) is selected. In addition, in the case where ITbis re qual to or larger than selected value of the coiltrol method (B), which is the predetermined threshold value, the control method (B) is selected. The selected value of the control method (C) and the selected value of the control method (H) can be set by deterinillation in adval~ccb, y past actual result value or simulation of - 24 - the rolling mill, or the like. When this optimum control method selection processing is executed on a step-like alteration point 1, a step-like alteration point 2, and a step-like alteration point 3 in the low speed, the medium speed, and the high speed shown in Fig. 15, in the case shown in Fig. 15, 5 such a result is obtained to select the control method (A) for the low speed, the control method (B) for the medium speed, and the control method (C) for the high speed, as an optimum control method. The control method selection apparatus 70 implements such a determination procedure of the optimum control method, and switches the control method to thus determined 10 optimum control method. In this case, because a control method of the entry-side TR 3 is different in the control method (A), the control method (B) and the control method (C), there may the case where switching cannot be executed during rolling operation. In that case, it is enough to continue rolling operation by the control method (A), and in the case when the material to be rolled having the same steel kind and the same plate width will come next time, 15 the control method is switched. Thus detesmined optimum control method is recorded in a database using steel kind of the material to be rolled, exit-side plate thickness and rolling speed as search conditions, and in the case of rolling the same kind of the material to be rolled next time, control is executed in accordance with the optimum control method recorded in the database. 2 0 Fig. 17 shows operation outline of the control output selection apparatus 70b. The control output selection apparatus 70b regards output from the screw-down plate thickness control apparatus 6 1, the speed plate thickness control apparatus 62, the speed tension control apparatus 63, and the screw-down tension control apparatus 64, and control method selection result from the optimum control method determination apparatus 70a, as input, and outputs 25 control command to the roll gap control apparatus 11, the entry-side TR speed command apparatus 65 and the entry-side TR control apparatus 32. As shown in Fig. 17, in the control output selection apparatus 70b, output from the screw-down plate thickness control apparatus 61, the speed plate thickness control apparatus 62, the speed tension control apparatus 63, and the screw-down tension control apparatus 64 is 30 input to gain controllers 8 1 to 84, respectively. The gain controllers 8 1 to 84 are signal adjusting units for outputting by multiplying gain to each o~rtpuot f the screw-down plate thickness coiltrol apparatus 61, the speed plate thickness control apparatus 62, the specd tension control apparatus 63, and the screw-down tension control apparatus 64. The gain of the gain controllers 8 1 to 84 is adjusted based on tlle control ~netl~osedl ection result from the optimum control method determination apparatus 70a. In the case of selection of the control method (A), output from the screw-down plate thickness control apparatus 61 is integral processed to output to the roll gap control apparatus 11. In addition, a torque-constant mode selection is output to the entry-side TR Ci control apparatus 32. Accordingly, gain of the gain controllers 82 to 84 is set to zero, by the control method selection result by the optimum control method determination apparatus 70a, as well as gain of the gain controller 81 is adjusted and set so that output from the screw-down plate thickness control apparatus 61 is integral processed by an integral processing unit 85. In addition, based on the control method selection result by the optimum control method 10 determination apparatus 70a, torque-constant control mode selection is output to the entry-side TR control apparatus 32. In the case of selection of the control method (B), output from the screw-down plate thickness control apparatus 61 is integral processed to output to the roll gap control apparatus 1 1, as well as output from the speed tension control apparatus 63 is integral processed 15 to output to the entry-side TR speed command apparatus 65. Accordingly, gain of the gain controllers 82 and 83 is set to zero, based on the control method selection result by the optimum control method determination apparatus 70a, as well as gain of the gain controllers 8 1 and 84 is adjusted and output from the screw-down plate thickness control apparatus 61 is integral processed by the integral processing unit 85, as well as it is set so that output from the speed 20 tension control apparatus 63 is integral processed by the integral processing unit 86. In the case of selection of the control method (C), output from the speed plate thickness control apparatus 62 is integral processed to output to tl~cen try-side TR speed command apparatus 65, as well as output from the screw-down tension control apparatus 64 is integral processed to output to the roll gap control apparatus 11. Accordingly, gain of the gain 25 controllers 8 1 and 84 is set to zero, based on the control method selection result by the optimum control method determination apparatus 70a, as well as gain of the gain controllers 82 and 83 is adjusted and output from the screw-down tension control apparatus 64 is integral processed by the integral processing unit 85, as well as it is set so that output from the speed plate thickness control apparatus 62 is integral processed by the integral processing unit 86. 30 That is, a control path connecting to the integral processing unit 85 and the roll gap control apparatus 11 functions as a roll gap control unit. And, by gain setting of the gain controllers 8 1 and 82, switching is made on roll gap control, based on which of tensioil of the material to be rolled, and plate thicl