W6832 - 1 - BACKGROUND OF THE INVENTION The invention relates to a plant control apparatus, a plant control method, and a plant control program and, more particularly, to an adjustment of a control gain in a state where a state amount deviation is large. 5 As shown in Fig. 17, plant control is performed by a method whereby a state amount result of a control subject plant 300 is detected or predicted by some means and controlled by a control apparatus 301 so as to coincide with a state amount instruction. As a control apparatus 301, generally, PID (Proportional Integral Derivative) control is made. Although the PID control includes proportional control, integral control, and derivative control, 10 actually, necessary functions are combined and used in accordance with characteristics of the control subject plant 300 or characteristics of control which is needed. Although various kinds of plants exist as control subject plants serving as subjects of the plant control, for example, there is a hot tandem mill. In the hot tandem mill, a tension and a rolling load which are applied to a material to be rolled (hereinbelow, referred to as a roll 15 material) are controlled by using a roll gap serving as an interval between upper and lower work rolls and rolling speeds of equipment before and after the mill, thereby performing the rolling operation. A looper for supporting the roll material between mill stands is disposed between the mill stands. The looper pushes the roll material upward between the stands, so that a 20 tension of the roll material changes. The tension of the roll material can be detected by measuring a pressure of the roll material applied to the looper. A height of roll material which is supported by the looper is controlled so as to become a desired height by a pressure applied by a hydraulic cylinder. In this control, the proportional integral control is used. In order to prevent a decrease in plate width caused by a loose of the roll material between the mill stands or 25 by an overtension, tension control is necessary. For this purpose, the tension control is performed by using the proportional integral control. As a technique of such a control system, a method whereby precedent control by an open loop and the proportional integral feedback control are switched and used has been proposed (for example, refer to JP-A-9-209712). According to the technique disclosed in JP-A- 30 9-209712, by using the precedent control by the open loop, difficulty of gain setting regarding load trackability in the feedback control is avoided and, if a deviation lies within a predetermined W6832 - 2 - range, by switching the control from the open loop to the feedback control, a deterioration in load trackability is prevented. However, since it is a prerequisite condition that when the deviation is large, the feedback control is not used, its essence differs from the present invention using the feedback control as a prerequisite. 5 SUMMARY OF THE INVENTION In the control subject plant such as production facilities or the like in a factory in the plant control, there is a case where the operation has to be stopped when the control becomes abnormal. When such a situation occurs, since a production activity in the control subject plant stops, an influence which occurs increases, that is, there is a case where a product cannot be 10 provided to the customer, an influence is exerted on the production of subsequent steps, or the like. Therefore, an abnormality of the control in the plant control has to be avoided. For example, in the hot tandem mill, since the roll materials of various kinds of product specifications are rolled, the characteristics of the control subject plant 300 become variable and the control gain of the control apparatus 301 also become variable. Therefore, it is 15 necessary to set the control gain in accordance with the product specification. However, since a model of a rolling phenomenon serving as a control subject plant 300 and parameters such as deformation resistance, coefficient of friction, and plate temperature which are used for the model are inaccurate, there is such a problem that an error of the control gain setting to the control apparatus 301 is large. 20 Fig. 18 shows a response difference due to the control gain in the case where the state amount instruction is changed in a step-like manner. As shown in Fig. 18, when the control gain setting to the plant control apparatus 301 is improper, it takes a time until the state amount result coincides with the state amount instruction (in the case where the control gain is too small) or the state amount is oscillated or diverged (in the case where the control gain is too 25 large). In any of those cases, an influence is also exerted on product quality in the the control subject plant. Therefore, like a case of newly starting up the control subject plant 300, a case of starting the production of new products, or the like, when a state of the control subject changes and a state amount deviation increases as a result, it takes a long time to adjust the control 30 apparatus 301 and there is also a possibility that a production stop of the control subject plant is caused or defective products are caused due to the defective control that is caused by the excessive or insufficient control gain. A subject to be solved by the invention is to provide a plant control method and a I W6832 - 3 - plant control apparatus in which in the case where a control subject plant is newly started up or the production of new products is started, the occurrence of an operation abnormality such as defective products or production stop due to an excessive or insufficient control gain of the control apparatus 301 is prevented, and at the same time, the control gain of the control 5 apparatus 301 can be automatically adjusted. In correspondence to the foregoing problems, it is an object of the invention to preferably perform a gain setting at the time when a state of a control subject changes largely in a feedback control system. According to an aspect of the invention, there is provided a plant control 10 apparatus for performing feedback control on the basis of a state amount deviation of a control subject plant, comprising: a first control unit for performing the feedback control of at least one of proportional control, integral control, and derivative control based on the state amount deviation; a second control unit for performing the integral control of a control gain lower than a control gain by the first control unit on the basis of the state amount deviation; a control 15 switching unit for, when an absolute value of the state amount deviation is equal to or less than a predetermined value, restricting the feedback control by the first control unit and allowing the second control unit to execute the integral control; and a gain adjusting unit for adjusting the control gain of the first control unit on the basis of a control response of the control subject plant to the integral control by the second control unit. 20 According to another aspect of the invention, there is provided a plant control method of switching a first control unit for performing feedback control of at least one of proportional control, integral control, and derivative control based on a state amount deviation of a control subject plant and a second control unit for performing the integral control of a control gain lower than a control gain by the first control unit on the basis of the state amount deviation 25 and controlling the control subject plant, comprising the steps of: when an absolute value of the state amount deviation is equal to or less than a predetermined value, restricting the feedback control by the first control unit and allowing the second control unit to execute the integral control; and adjusting the control gain of the first control unit on the basis of a control response of the control subject plant to the integral control by the second control unit. 30 According to another aspect of the invention, there is provided a plant control program for switching a first control unit for performing feedback control of at least one of proportional control, integral control, and derivative control based on a state amount deviation of a control subject plant and a second control unit for performing the integral control of a control gain lower than a control gain by the first control unit on the basis of the state amount deviation s j x W6832 - 4 - and controlling the control subject plant, wherein the plant control program allows an information processing apparatus to execute: a step of, when an absolute value of the state amount deviation is equal to or less than a predetermined value, restricting the feedback control by the first control unit and allowing the second control unit to execute the integral control; and a 5 step of adjusting the control gain of the first control unit on the basis of a control response of the control subject plant to the integral control by the second control unit. By using the invention, the gain setting when the state of the control subject changes largely in the feedback control system can be desirably performed. Other objects, features and advantages of the invention will become apparent 10 from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a whole construction of a rolling apparatus according to an embodiment of the invention. 15 Figs. 2 A and 2B are diagrams showing the operation of a looper in the rolling apparatus according to the embodiment of the invention. Fig. 3 is a diagram showing a construction of feedback control in a general rolling apparatus. Fig. 4 is a diagram showing a frequency response of a control gain in the general rolling apparatus. 20 Figs. 5 A, 5B and 5C are diagrams showing a convergence, an oscillation, and a divergence of a state amount due to a difference of the control gains, respectively. Fig. 6 is a diagram showing a construction of plant control in the embodiment of the invention. Fig. 7 is a diagram showing a construction of feedback control in the rolling apparatus according to the embodiment of the invention. 25 Fig. 8 is a diagram showing a setting form of a control gain according to the embodiment of the invention. Figs. 9 A, 9B and 9C are diagrams showing a response of a state amount due to a difference of the control gains according to the embodiment of the invention. Fig. 10 is a diagram showing a frequency response of the control gain in the rolling apparatus 30 according to the embodiment of the invention. Figs. 11A and 11B are diagrams showing a response of a state amount in the case where a steady-state deviation compensation according to the embodiment of the invention. Fig. 12 is a diagram showing a form of tension control according to the embodiment of the W6832 - 5 - invention. Fig. 13 is a diagram showing a form of tension control according to the embodiment of the invention. Fig. 14 is a diagram showing a form of tension control according to the embodiment of the 5 invention. Fig. 15 is a diagram showing a construction about a learning of the control gain according to the embodiment of the invention. Fig. 16 is a diagram showing a hardware construction of an information processing apparatus constructing a plant control apparatus according to the embodiment of the invention. 10 Fig. 17 is a diagram showing a construction of plant control according to a related art. Fig. 18 is a diagram showing a response of a state amount deviation due to a difference of control gains according to the related art. DESCRIPTION OF THE EMBODIMENTS Embodiment 1 15 An embodiment of the invention will be described hereinbelow with respect to tension control between mill stands in a hot mill as an example. Fig. 1 is a diagram showing a tension control system according to the embodiment. As shown in Fig. 1, in an interstand tension control unit 10, a tension which is applied to a roll material 8 existing between an i-1 stand rolling mill 1 and an i stand rolling mill 2 of a hot tandem mill is detected by a tension 20 meter 9 disposed in a looper 7 and a speed instruction to an i-1 stand speed control apparatus 11 is changed, thereby controlling a rolling speed of the i-1 stand rolling mill 1. The looper 7 is constructed by: a looper arm 15 which can be rotated around a looper fulcrum 14 existing at a position which is mechanically fixed; a hydraulic cylinder 13 for changing a position of a looper roll 16 by rotating the looper arm 15 around the looper fulcrum 25 14; and a cylinder position detector 17 for detecting a cylinder position. By pushing the roll material 8 upward, the looper roll 16 receives the tension which is applied to the roll material 8. By measuring the force which is applied to the looper roll 16 by the tension meter 9, the interstand tension control unit 10 obtains the tension which is applied to the roll material 8. The operation of the looper 7 is shown in Figs. 2 A and 2B. When a front edge 30 portion 30 of the roll material exists between the i-1 stand rolling mill 1 and the i stand rolling mill 2, if the looper roll 16 collides with the roll material front edge portion 30, a machine is broken. Therefore, the looper roll 16 is on standby at a position lower than a passing position of the roll material 8 as shown in Figs. 2A. When the roll material front edge portion 30 W6832 - 6 - reaches the i stand rolling mill 2, the looper roll 16 is moved to such a position that the roll material 8 is lifted up as shown in Figs. 2B, so that the tension which is applied to the roll material 8 can be measured by the tension meter 9. Since the tension of the roll material 8 is propagated from the looper roll 16 to the 5 hydraulic cylinder 13 through the looper arm 15, when the tension of the roll material 8 fluctuates, a difference occurs between this tension and a pressure of the hydraulic cylinder 13 and the cylinder position changes. Thus, the position of the looper roll 16 changes. Since the position fluctuation of the looper roll 16 becomes a tension fluctuation and also exerts an influence on the stability of the rolling operation, looper position control adapted to make the 10 position constant is made. A looper position control apparatus 20 operates and controls the pressure of the hydraulic cylinder 13 by using the cylinder position measured by the cylinder position detector 17 so that the position of the looper roll 16 becomes constant. Fig. 3 is a block diagram showing a construction of interstand tension control in the hot tandem mill in the related art. The interstand tension control unit 10 outputs a control 15 instruction to the i-1 stand speed control apparatus 11 by using the proportional integral control so as to eliminate a deviation between a tension instruction and a tension result, and changes an i-1 stand rolling speed. That is, in the embodiment, the interstand tension control unit 10 functions as a first control unit. As control by the first control unit, the feedback control by the proportional control, integral control, and derivative control can be used. When the i-1 stand 20 rolling speed changes, the tension result changes by a speed - tension response 31. This fluctuation is detected by the tension meter 9 and used as a tension result. The fluctuation of the tension result becomes a pressure fluctuation of the hydraulic cylinder 13 by a plate tension - cylinder pressure 32 serving as a mechanical system. However, when the pressure of the hydraulic cylinder 13 fluctuates, the cylinder position 25 changes, the looper position fluctuates by a looper mechanical system 35, and an interstand plate road length fluctuates eventually by a looper position - interstand plate road length 34. The interstand plate road length fluctuation becomes a tension fluctuation by a plate road length change - tension response 33 and the tension result fluctuates. The speed - tension response 31 is shown by the following equation (1). The plate road length change - 30 tension response 33 is shown by the following equation (2). W6832 - 7 - ^L = -J^_ (1) ^ _ »WS (2) ^dL- 1+Tf f-S The speed - tension response 31 and the plate road length change - tension response 33 are caused by a rolling phenomenon and change in dependence on a material, a plate thickness, a rolling speed, and the like (hereinbelow, called a rolling schedule) of the roll material 8. On the contrary, if their values are known, a response of the i-1 stand speed control 5 apparatus 11 can be measured and since it does not depend on the rolling schedule, a control gain of the interstand tension control unit 10 can be set as shown in Fig. 4. Generally, since it is considered that the order of Ta is equal to about a few ms, 1/TCT is larger than a response