DESCRIPTION WIND POWER GENERATOR SYSTEM AND CONTROL METHOD OF THE SAME Technical Field The present invention relates to a wind power generator system and method for controlling the same, particularly, to control of the output power and the pitch angle of a wind power generator system adopting a variable-speed and variable-pitch control method. Background Art One of the promising control methods for a wind power generator system is a variable-speed and variable-pitch control method, in which the rotational speed of the windmill rotor (that is, the rotational speed of the generator) is variable and the pitch angle of the blades is variable. Advantages of the variable-speed and variable-pitch control method include increased energy capture from the wind and decreased output fluctuation. With respect to the variable-speed and variable-pitch control method, it is of importance to optimize the control of the output power of the generator and the pitch angle of the blades. Japanese Translation of International Publication No. 2001-512804 discloses a control method in which the torque of the generator is controlled with a field orientation control while the pitch angle is controlled independently of the torque of the generator. In the disclosed control method, desired output power of the generator is determined In response to the rotational speed of the generator using a lookup table, and a torque command for the generator is determined from the desired output power, The torque of the generator is controlled by a field orientation control in response to the torque command, on the other hand, the pitch angle of the blades is controlled by PID control, PI control or PD control responsive to the deviation between the rotational speed of the generator and a desired rotational speed. One issue for the wind power generator systera is how to deal with occurrence of a transient wind null, that is, a short-time reduction in the wind speed, Generally, a wind power generator system is designed to generate rated power in the case where the rotational speed of the windmill rotor is equal to or larger than a rated rotational speed. In such a wind power generator system, the output power is reduced below the rated power, when the rotational speed of the windmill rotor is reduced below the rated rotational speed due to the occurrence of the transient wind null. This causes output power fluctuation and generation efficiency reduction. Disclosure of Invention Therefore, an object of the present invention is to provide a wind power generator system which suppresses output power fluctuation and generation efficiency reduction even when a transient wind null occurs. A wind power generator system according to the present invention includes; a windmill rotor including a blade having a variable pitch angle; a generator driven by the windmill rotor; and a control unit controlling the output power of the generator and the pitch angle of the blade in response to the rotational speed (a) of the windmill rotor or the generator. The control unit performs a first control in which the output power is controlled in accordance with a predetermined power-rotational speed curve until the rotational speed (©) increases and reaches a predetermined rated rotational speed, and performs a second control in which the output . power is controlled to a predetermined rated power when the rotational speed (©) exceeds the rated rotational speed; the control unit is responsive to the pitch angle for maintaining a state of performing the second control is or for switching to a state of performing the first control, when the rotational speed (o) is reduced below the rated rotational speed after the control unit is once placed into the state of performing the second control. Here, the pitch angle is an angle formed between a chord of the blade and a rotation plane of the windmill rotor. Namely, the windmill rotor extracts more energy from wind when the pitch angle is small, and the windmill rotor extracts less energy from the wind when the pitch angle is large. The wind power generator system configured as stated above can suppress the output power fluctuation by using the rotational energy of the windmill rotor when the wind speed is reduced only for a short time. This is because the wind power generator system according to the present invention keeps the output power at the predetermined rated power in response to the pitch angle of the blade, when said rotational speed (CD) is reduced below said rated rotational speed. When it is determined from the pitch angle of the blade that the system is in a state in which the output power can be kept at the predetermined rated power, the output power is kept at the rated power, and this allows effectively extracting the rotational energy of the windmill rotor and suppressing the output power fluctuation and the power generation efficiency reduction. Preferably, when the rotational speed (o) is reduced below the rated rotational speed after the control unit is once placed into the state of performing the second control, the control unit maintains the state of performing said second control for the case when the pitch angle is larger than a predetermined pitch angle, not switching to the state of performing the first control until the pitch angle reaches the predetermined pitch angle. Preferably/ the control unit controls said pitch angle in response to the difference between the rotational speed (ttt) of the windmill rotor or the generator and the predetermined rated rotational speed and the difference between the output power and the rated power. In this case, the control unit preferably controls the pitch angle to be reduced when the output power is lower than the rated power. The control unit preferably increases the output power of the generator in response to said rotational speed ((D) when a gust is detected. In the case where the wind power generator system further includes; a rotation mechanism rotating a direction of the rotational surface of the windmill rotor; and a wind direction detector detecting a windward direction and the windmill rotor includes a pitch drive nechanisra driving the blade, it is preferable that the control unit controls the rotation mechanism so as to move the rotation plane of the windmill rotor away from the windward direction when detecting a failure in the pitch drive mechanism. Preferably, the control unit controls reactive power outputted from the generator to a power grid connected to the generator in response to a voltage of the power grid, and controls the pitch angle in response to the reactive power. In the case where the wind power generator system further includes an emergency battery and a battery charger charging the emergency battery with power received from the power grid, wherein the windmill rotor includes a pitch drive mechanism driving the blade and wherein the emergency battery supplies power to the pitch drive mechanism and the control unit when the voltage of the power grid connected to the generator is reduced, the control unit preferably controls the output power to be increased while the emergency battery is being charged. A method of controlling a wind power generator system according to the present invention is a method of controlling a wind power generator system provided with! a windmill rotor including a blade having a variable pitch angle; and a generator driven by the windmill rotor. The control method includes a control step of controlling output power of the generator and a pitch angle of the blade in response to the rotational speed (oj) of the windmill rotor or the generator. Said control step includes steps of: (A) performing the first control in which said output power is controlled in accordance with to a predetermined power-rotational speed curve until said rotational speed (eo) increases to reach a predetermined rated rotational speed; (B) performing a second control in which said output power is controlled to a predetermined rated power when • said rotational speed (co) exceeds the rated rotational speed; and (C) in response to the pitch angle, maintaining the state of performing said second control or switching to the state of performing said first control, when said rotational speed (oa) is reduced below the rated rotational speed after the state of performing said second control is once established. The present invention provides the wind power generator system which can suppress the output power fluctuation and the generation efficiency reduction even when a transient wind null occurs. Brief Description of Drawings FIG. 1 is a side view showing the configuration of a wind power generator system in one embodiment of the present invention; FIG. 2 is a block diagram showing the configuration of a pitch drive mechanism of the wind power generator system of the present embodiment/ FIG. 3 is a block diagram showing the configuration of the wind power generator system of the present embodiment; FIG, 4 is a graph showing a power control method performed by the wind power generator system of the present embodiment; FIG. 5 is a block diagram showing an example of the configuration of a main control unit of the wind power generator system of the present embodiment; FIG. 6 is a table explaining operations performed by a power controller and a pitch controller of the wind power generator system of the present embodiment; FIG. 7 is a graph showing an example of an operation performed by the wind power generator system of the present embodiment; FIG. 8 is a block diagram showing another configuration of the wind power generator system of the present embodiment; FIG. 9 is a flowchart of a preferred control performed by the wind power generator system of the present embodiment; FIG. 10 is a flowchart of another preferred control performed by the wind power generator system of the present embodiment; FIG. 11 is a flowchart of still another preferred control performed by the wind power generator system of the present embodiment; and FIG. 12 is a flowchart of still another preferred control performed by the wind power generator system of the present embodiment. Best Modes for Carrying Out the Invention A wind power' generator system according to the present invention will be described hereinafter in detail with reference to the attached drawings. FIG. 1 is a side view showing the configuration of a wind power generator system 1 in one embodiment of the present invention. The wind power generator system 1 is provided with a tower 2 and a nacelle 3 provided on the top end of the tower 2. The nacelle 3 is rotatable in the yaw direction and directed to a desired direction by a nacelle rotation mechanisrti 4. Mounted in the nacelle 3 are a wound-rotor induction generator 5 and a gear 6. The rotor of the wound-rotor induction generator 5 is connected to a windmill rotor 7 through the gear 6. The nacelle 3 additionally includes an anemometer 10. The anemometer 10 measures the wind speed and the wind direction. As described later, the nacelle 3 is rotated in response to the wind speed and the wind direction measured by the • anemometer 10. The windmill rotor 1 includes blades 8 and a hub 9 supporting the blades 8. The blades 8 are provided so that the pitch angle thereof is variable. More specifically, as shown in FIG. 2, the hub 9 contains therein hydraulic cylinders 11 driving the blades 3 and servo valves 12 supplying hydraulic pressure to the hydraulic cylinders 11. The hydraulic pressure supplied to the hydraulic cylinders 11 is controlled by the openings of the servo valves 12, thereby controlling the blades 8 to a desired pitch angle, FIG. 3 is a block diagram showing details of the configuration of the wind power generator system 1. The wind power generator system 1 in this embodiment is a sort of doubly-fed variable speed wind turbine system. Namely, the wind power generator system 1 of this embodiment is configured to output the power generated by the wound-rotor induction generator 5 to the power grid 13 from both of the stator and rotor windings. Specifically, the wound-rotor induction generator 5 has the stator winding directly connected to the power grid 13 and the rotor winding connected to the power grid 13 through an AC-DC-AC converter 17. The AC-DC-AC converter 17, which includes an active rectifier 14, a DC bus 15 and an inverter IS, converts AC power received from the rotor winding into AC power adapted to the frequency of the power grid 13. The active rectifier 14 converts the AC power generated by the rotor winding into DC power and outputs the DC power to the DC bus 15. The inverter 16 converts the DC power received • from the DC bus 15 into AC power of a frequency equal to that of the power grid 13 and outputs the AC power to the power grid 13. The output power which the wound-rotor induction generator 5 outputs to the power grid 13 is controlled by the active rectifier 14 and the inverter 16. The AC-DC-AC converter 17 also has a function of converting AC power received from the power grid 13 into AC power adapted to the frequency of the rotor winding, and the AC-DC-AC converter 17 is used to excite the rotor winding, depending on the operating state of the wind power generator system 1, In this case, the inverter 16 converts the AC power into the DC power and outputs the DC power to the DC bus 15. The active rectifier 14 converts the DC power received from the DC bus 15 into the AC power adapted to the frequency of the rotor winding and supplies the AC power to the rotor winding of the wound^rotor induction generator 5. A control system of the wind power generator system 1 includes a PLG (pulse logic generator) 18, a main control unit 19, a voltage/current sensor 20, a converter drive control unit 21, a pitch control unit 22, and a yaw control unit 23. The PLG 18 measures the rotational speed © of the wound-rotor induction generator 5 (hereinafter, referred to as the generator rotational speed ©). The voltage/current sensor 20, which is provided on power lines connected between the wound-rotor induction generator 5 and the power grid 13, measures the voltage Vyrn of the power grid 13 ("grid voltage") and the output current I^^^^ I outputted from the wound-rotor induction generator 5 to the power grid 13.■ The main control unit 19 generates an real power command P% a reactive power command Q* and a pitch angle command p', in response to the generator rotational speed © measured by the PLG 18, and also generates a yaw command in response to the wind speed and the wind direction measured by the anemometer 10. As described later in detail, one feature of the wind power generator system 1 of this embodiment is a control algorithm for generating the real power command P* and the pitch angle command P*. The converter drive control unit 21 controls real power P and reactive power Q outputted to the power grid 13 in response to the real power command P* and the reactive power command Q*, respectively. The converter drive control unit 21 also controls the turn-on-and-off of power transistors within the active rectifier 14 and the inverter 16. Specifically, the converter drive control unit 21 calculates the real power P and the reactive power Q to be outputted to the power grid 13 from the voltage Vgji,^ and the output current Igrid measured by the voltage/current sensor 20, Further, the converter drive control unit 21 generates PWM signals for the PWM control in response to ■ the difference between the real power P and the real power command P* and the difference between the reactive power Q and the reactive power command Q% and supplies the generated PWM signals to the active rectifier 14 and the inverter 16. The real power P and the reactive power Q outputted to the power grid 13 are thereby controlled. The pitch control unit 22 controls the pitch angle p of the blades 8 in response to the pitch angle command p* transmitted from the main control unit 19, The pitch angle P of the blades 8 is controlled to coincide with the pitch angle command 3*. The yaw control unit 23 controls the nacelle rotation mechanism 4 in response to the yaw command transmitted from the main control unit 19. The nacelle 3 is oriented to the direction indicated by the yaw command. An AC/DC converter 24 is connected to the power lines connected between the power grid 13 and the wound-rotor induction generator 5. The AC/DC converter 24 generates DC power from AC power received from the power grid 13. The AC/DC converter 24 supplies the DC power to the control system of the wind power generator system 1, particularly to the servo valves 12, the main control unit 19, and the pitch control unit 22 used to control the pitch angle p of the blades 8. Moreover, the wind power generator system 1 is provided with an uninterruptible power supply system 26 which includes a battery charger 27 and an emergency buttery 28 so as to stably supply DC power to the servo valves 12/ the main control unit 19, and the pitch control unit 22. From requirements of wind power generator system standards, it is necessary for the wound-rotor induction generator 5 to remain connected to the power grid 13 even when the grid voltage Vg^jd falls. This requires appropriately controlling the pitch angle of the blades 8 to thereby maintain the rotational speed of the wound-rotor induction generator 5 to a desired value even when the voltage of the power grid 13 falls. To satisfy such requirements, when the grid voltage Vg^id falls to a predetermined voltage, the uninterruptible power supply system 26 is connected to the servo valves 12, the main control unit 19/ and the pitch control unit 22 by a switch 25, to supply power from the emergency battery 28 to the servo valves 12, the main control unit 19, and the pitch control unit 22 'P*^* «^ ^-r-K snnle of the blades 8 is thereby kept controlled. The emergency battery 28 is connected to the battery charger 27. The battery charger 27 charges the emergency battery 28 with the DC power supplied from the AC/DC converter 24. One feature of the wind power generator system 1 of this embodiment is optimized control of the output power P of the wound-rotor induction generator 5, FIG. 4 is a graph showing the relationship between the real power ' command P* and the rotational speed Gt of the wound-rotor induction generator S, depicting a method of controlling the output power P performed, by the wind power generator system 1 of this embodiment. When the generator rotational speed ra is smaller than a minimum rotational speed a3„i„, the real power command P* > for the wound-rotor induction generator 5 is controlled to zero. The minimum rotational speed o„i„ is a minimum rotational speed at which power can be generated by the wound-rotor induction generator 5, and the minimum rotational speed co„,i„ is determined in accordance with characteristics of the wind power generator system 1. When the generator rotational speed co is larger than the minimum rotational speed a„i„, the real power command P* is controlled in one control mode selected from two control modes; an optimum curve control mode and a rated value control mode. In the optimum curve control mode, the real power command P* is controlled to coincide with an optimizedpower Popt defined by the following Equation (1): P,pt = K(»% (1) where K is a predetermined constant. It is known that it is optimum for the windpower generator system 1 to control the output power to be proportional to the cube of the generator rotational speed. Accordingly, the output power P is controlled to be proportional to the cube of the generator rotational speed a in the first control mode. The optimum curve control mode is used mainly in a range in which the generator rotational speed <» is larger than the miniinum rotational speed o„i„ and smaller than a rated • rotational speed o^^^. Note that the rated rotational speed o„ax is the rotational speed at which the wound-rotor induction generator 5 operates in the steady operation. The generator rotational speed© is controlled to the rated rotational speed CD^SX ^^^ possible) by controlling the pitch angle of the blades 8. In the rated value control mode/ on the other hand, the output power P is controlled to the rated power Prctea* The rated value control mode is used mainly in a range in which the generator rotational speed © is equal to or larger than the rated rotational speed (»„»x • ^^ a steady condition in which wind blows with a rated wind speed, the generator rotational speed o is controlled to the rated rotational speed <»„jx and the output power P is controlled to the rated power P„i,d. An important feature of the wind power generator system 1 of this embodiment is in a fact that switching from the rated value control mode to the optimum curve control mode is made in response to the pitch angle p of the blades 8. When the generator rotational speed © is increased to reach the rated rotational speed %^^, the power control is unconditionally switched from the optimum curve control mode to the rated value control mode. When the generator rotational speed o) is decreased below the rated rotational speed (o„j,x? on the other hand/ the pitch angle p is first reduced. The power control is not switched from the rated value control mode to the optimum curve control mode until the pitch angle 0 reaches a minimum value 3„in, Namely, the real power command P* is switched from the rated power Pratsa to the optimized power value Popt* ^^ ■ other words, the real power command P* is kept at the rated power Prated unless the pitch angle p reaches the minimum value J5„ifi (that is, the pitch angle command P* reaches the minimum value p^iJ . It should be noted that the fact that the pitch angle P is set to the minimum angle p^i^ implies I that the output coefficient of the windmill rotor 7 is maximum with the pitch angle p set to the fine-side limit value, since the pitch angle p is the angle formed between the chords of the blades 8 and the rotation plane of the windmill rotor. The control in which the output power P is kept at the rated power P^t^^ until the pitch angle p reaches the minimum value P„i„ is effective for suppressing output power fluctuation and avoiding generation efficiency reduction when a transient wind null occurs. Under the above-described control, the real power command P* is kept at the rated power P^ted when the generator rotational speed o is reduced below the rated rotational speed %^^ if such state continues only for a short time, and thereby the fluctuation in the output power P is suppressed. Furthermore, the wind power generator system 1 of this embodiment allows making effective use of rotational • energy of the windmill rotor 7, effectively improving the generation efficiency, since the output power P is not reduced from the rated power P^tei until the increase in the output coefficient of the windmill rotor 7 through the reduction in the pitch angle 3 becomes impossible, when the generator rotational speed to is reduced below the rated rotational speed o„^^. It should be noted that the power control is switched from the rated value control mode to the optimum curve control mode irrespectively of the pitch angle P (or the pitch angle command 3*), when the generator rotational speed © is reduced below an intermediate rotational speed a>M (= (Oratn + «ii.aJ / 2) . It is unprsferable for securing the control stability to maintain the output power P at the rated power Ptat«d when the generator rotational speed © is excessively small. FIG. 5 is a block diagram showing an example of a configuration of the main control unit 19 for realizing a control shown in FIG. 4. It should be noted that FIG. 5 shows only one example of the configuration of the main control unit 19; the main control unit 19 may be impleinented as hardware, software, or a combination of hardware and software. The main control unit 19 includes a power control module 31 generating the real power command P* and the reactive power command Q* and a pitch control module 32 generating the pitch angle command (3'. The power control module 31 includes a selector 33, a subtracter 34, a PI controller 35, a power limiter 36, and a power setting calculator 37. On the other hand, the pitch control module 32 includes a subtracter 38, a PI controller 39, a subtracter 40, a PI controller 41, and an adder 42. The selector 33, the subtracter 34, the PI controller 35, the power limiter 36, the power setting calculator 37, the subtracter 38, the PI controller 39, the subtracter 40, the PI controller 41, and the adder 42 perform respective calculation steps synchronously with a clock used in the main control unit 19, and the real power command ?*, the reactive power command Q', and the pitch angle command j3* are thereby generated. In detail, the selector 33 selects one of the minimum rotational speed oo„i„ and the rated rotational speed co„ax as a power control rotational speed command (Dp* in response i to the generator rotational speed co. More specifically, the selector 33 sets the power control rotational speed command dOp* to the minimum rotational speed t»aiB, when the generator rotational speed a is equal to or smaller than the intermediate rotational speed (D„, and sets the power control rotational speed command cOp* to the rated rotational speed oo^a^, when the generator rotational speed (0 is larger than the intermediate rotational speed »„. The subtracter 34 calculates the deviation Aop by subtracting the power control rotational speed cammand cttp* • from the generator rotational speed co. The PI controller 35 performs PI control in response to the deviation Aop to generate the real power command P*. Note that the range of the generated real power command P* is limited by an power command lower limit P^i„ and an power command upper limit P„ax supplied from the power limiter 36. Namely, the real power command P* is limited to be equal to or higher » than the power coitimand lower limit P^i^ and equal to or lower uppeT than power command lQM&-r limit P^sx- The power limiter 36 determines the power command lower limit P„i„ and the power command upper limit P^^^ supplied to the PI controller 35 in response to the generator rotational speed © and the pitch angle command P*. Further, the power limiter 36 supplies the rated power Pjcated to the subtracter 4 0 of the pitch control module 32. As described later, the power control shown in FIG, 4 is implemented by appropriately determining the power 3- command lower limit P„i„ and the power command upper limit P„„ generated by the power limiter 36 as well as the power control rotational speed command co/ determined by the selector 33. The power setting calculator 37 generates the reactive power command Q* from the real power command P* generated by the PI controller 35 and a power factor command indicating a power factor of the AC power outputted from the wind power generator system I, and outputs the real power command P* and the reactive power command Q*. As described above, the real power command P* and the reactive power command Q* are used to control the real power P and the reactive power Q outputted from the wind power . generator system 1, respectively. The subtracter 38 of the pitch control module 32 calculate the deviation AOp by subtracting a pitch control rotational speed command (Hp" from the generator rotational speed ffi. The pitch control rotational speed command cop* is coincident with the rated rotational speed ffl„a^, and therefore the deviation Aosp represents the difference between the generator rotational speed © and the rated rotational speed (H^ax' The PI controller 39 performs PI control in response to the deviation A©g to generate a pitch angle command baseline value Pic*. The pitch angle command baseline value Pin' mainly controls the finally generated pitch angle command (5*, but the pitch angle command baseline value p^/ does not always coincide with the pitch angle command p*. The pitch angle command baseline value Pi/ is determined so that the generator rotational speed oo is controlled to the rated rotational speed o^^^. The subtracter 40 generates a deviation AP by subtracting the rated power Px«,ted fJ^^^^ the real power comiaand P*. The PI controller 41 performs the PI control in response to the deviation AP to generate a correction value Ap'. The adder 42 adds up the pitch angle command baseline value 3i„* and the correction value Ap* to generate the pitch angle command |J'. The subtracter 40 and the PI controller 41 of the pitch control module 32 have a role to prevent the pitch control module 32 from undesirably interfering with the power control when the generator rotational speed » increases up to the rated rotational speed Og,^^ and the power control is switched from the optimum curve control mode to the rated value control mode. The PI controller 39 of the pitch control module 32 is designed to adjust the generator rotational speed o to the rated rotational speed ©pax* This may result in that the aerodynamic energy to be extracted as the power is undesirably abandoned. Therefore, in this embodiment, the PI controller 41 generates the correction value Ap* in response to the difference between the rated power P„tB4 and the real power command P*, and the pitch angle command p* is corrected with the correction value Ap'. The correction value Ap* is determined so that the pitch angle command p' is smaller than the pitch angle command baseline value P^/, i.e., the pitch angle p is set closer to the fine-side limit value, when the real power command P' is lower than the rated power command Prated/ i.e.^ the deviation AP (= P* - Prated) is negative. Such control allows avoiding the pitch angle P from being closer to the feather-side limit value just before the generator rotational speed co reaches the rated rotational speed cOmax* After the generator rotational speed oo reaches the rated rotational speed &„^^, the deviation AP becomes zero and the correction value Ap* becomes zero. FIG. 6 is a table showing operations performed by the power control module 31 and the pitch control module 32 of the main control unit 19. The operations performed by the power control module 31 and the pitch control module 32 will be described for the following five cases: Case (1): The generator rotational speed © is equal to or I larger than the minimum rotational speed ©min and equal to or smaller than the intermediate rotational speed o(»„ (- («>min + 0)„,J / 2) . In the case (1), the power control rotational speed command <»?* is set to the minimum rotational speed ffl„in by the selector 33, and the power command lower limit P„in and the power command upper limit ?^sx are set to zero and P^p^ (= K©*) , respectively. Besides, the real power command P* is always set to the power command upper limit P„ax/ since the deviation Aa^ (= © - ca^^J is positive and the generator rotational speed © is controlled to' the rated rotational speed ©aa„, The real power command P' is eventually set to the optimized power value P^ptt since the power command upper limit P„aj is P^pt. In other words, the power control is set to the optimum curve control mode. In this case, the pitch angle command j3* is eventually set to the fine-side limit value, i.e., the itiinimum pitch angle p„i„/ since the pitch control module 32 controls the generator rotational speed a to the rated rotational speed c»„a;;. Case (2); The generator rotational speed ea exceeds the intermediate rotational speed a„, whereby the generator rotational speed m is set in a range where the generator rotational speed © is larger than the intermediate rotational speed IBK and smaller than the rated rotational speed CD;,,,. In the case (2) , the power control rotational speed command ©/ is set to the rated rotational speed a^^^ by the selector 33, and the power command lower limit P„in and the power command upper limit P„aj, are set to P(,pt and Prated/ respectively, in this case, the real power command P* is always set to the power command lower limit P„in, since the deviation Afflp (= oj - a>„aJ is negative and the generator rotational speed ra is controlled to the rated rotational speed (Oniax by the pitch control module 32. The real power command P* is eventually set to the optimized power value Popt, since the power command lower limit ^j^ is Popt • In other words, the power control is set into the optimum curve j control mode. The above-described correction of the pitch angle command p' with the correction value 40* validly works in the case (2). In the case (2), since the real power command P* is lower than the rated power Prattd' the deviation AP is negative and the correction value Ap' is therefore negative. Accordingly, the pitch angle command p* is reduced below the pitch angle command baseline value pjn*/ that iS/ the pitch angle P is set closer to the fine-side lower limit, This allows converting the aerodynamic ■ energy into the power more effectively. Case (3): The generator rotational speed (o is equal to or larger than the rated rotational speed CD„„ . In the case (3), the power control rotational speed command «>/ is set to the rated rotational speed a„ax by the selector 33, and the power command lower limit P^j„ and the power command upper limit P„a^ are both set to Pxated' Therefore, the real power command P* is set to the rated power Plated* 1^1 Other words, the power control is set into the rated value control mode. On the other hand, the pitch angle command P' is controlled by the PI control so that the generator rotational speed © is equal to the rated rotational speed o^a,,' Case (4): The generator rotational speed to is reduced below the rated rotational speed o^ax/ whereby the generator rotational speed m is set in a range larger than the intermediate rotational speed ffl„ and smaller than the rated rotational speed co^j^, while the pitch angle )3 does not reach the minimum pitch angle Pnin* In the case (4), the power control rotational speed command cop' is set to the rated rotational speed to^^x by the selector 33. Additionally, the power command lower limit P„in is set to a smaller one of the one-operation-step previous real power command P* and the power command upper limit Pfcsx at the current operation step, and the power command upper limit P^^^ is set to the rated power Pratsd* As a result, the real power command P* is set to the rated power Prated- '^^ othsr words, the power control is set into the rated value control mode even when the generator rotational speed © is reduced below the rated rotational speed a)„ex. Whether or not the pitch angle 3 reaches the minimum pitch angle |3„j„ is determined based on whether the pitch angle command |3' coincides with the minimum pitch angle (5„i„. The pitch angle command P* is controlled by PI control so that the generator rotational speed oo is set to the rated rotational speed comax- ^^ the case (4), the generator rotational speed a> is smaller the rated rotational speed t until the generator rotational speed o reaches the rated rotational speed (^^^K after the wind power generator system 1 starts operating (the above-described case (2)). Accordingly, the outputted real power P is increased as the generator rotational speed