WIND TURBINE GENERATOR AND ITS CONTROL METHOD BACKGROUND OF THE INVENTION Technical Field The present invention relates to a wind turbine generator and a method of controlling the wind turbine generator, and more particularly, to the yaw control of a wind turbine generator. Description of the Related Art/ Background Art One important control for improving the efficiency of a wind turbine generator is yaw control for controlling the direction of the wind turbine rotor so that the direction of the wind turbine rotor corresponds to the wind direction. When the wind turbine rotor faces into the wind, the wind turbine generator achieves maximum efficiency. Accordingly, the direction of the wind turbine rotor needs to be controlled by performing yaw rotation of a nacelle, in which the wind turbine rotor is mounted, in accordance with a wind direction. There have been various challenges for a yaw rotating mechanism or a yaw control technique. For example, Japanese Patent Application Laid-Open No. 2004-285858 discloses a technique that detects the wind direction and wind power by a laser-type wind vane/anemometer and performs yaw control on the basis of the detected wind direction and wind speed. Further, Japanese Patent Application Laid-Open No. 2005-113899 discloses the configuration of a drive mechanism for performing the yaw rotation of the nacelle. One of the important aspects of yaw control for a wind turbine generator is to reduce the number of yaw rotations of the nacelle. The nacelle is heavy and, for this reason, if the number of yaw rotations of the nacelle is large, the mechanical load applied to the rotating mechanism for rotating the nacelle or the braking mechanism for stopping nacelle rotation increases. As a result, mechanical wear on these mechanisms increases. It is preferable that the number of yaw rotations be minimized in order to reduce wear on the rotating mechanism or the braking mechanism. The control logic used to meet these demands for the most common yaw control is a control logic performing yaw rotation of the nacelle so that deviation from the wind direction becomes zero (that is, the orientation of the wind turbine corresponds to the newest wind direction) when a state where the absolute value of the deviation (the wind direction deviation) between the orientation of a wind turbine (that is, the direction of a wind turbine rotor) and the actual wind direction is larger than a predetermined threshold continues for a predetermined duration (for example, 20 seconds) as shown in Fig. 18. Unless the absolute value of the wind direction deviation exceeds a threshold, yaw rotation is not performed in this control logic. Accordingly, it may be possible to reduce the number of yaw rotations by appropriately setting the threshold. As shown in Fig. 19, one problem with this control logic is that, in a situation where the wind direction changes gradually over a long time (typically, several hours), the magnitude of the wind direction deviation is not reduced on average. In general, the wind at a certain location has high degree of turbulence during the day and the wind direction changes at random, but it is often the case that at night the wind direction does not randomly change. In other words, the wind situation often changes at night so that the wind direction changes over an extended period of time. According to the above-mentioned control logic, the magnitude in the wind direction deviation approaches zero on average in situations where the degree of turbulence is high and the wind direction changes at random. However, if the wind direction changes gradually over a long time (typically, several hours) as shown in Fig. 19 (A of Fig. 19), the wind direction deviation becomes zero only momentarily (C of Fig. 19) in the above-mentioned control logic even though the yaw rotation is repeated (B of Fig. 19). Accordingly, the average magnitude of the wind direction deviation is not reduced. This is not preferable for improving the efficiency of the wind turbine generator. SUMMARY OF THE INVENTION Accordingly, an overall object of the invention is to improve an efficiency of a wind turbine generator, specifically, an object of the invention is to achieve at least one of the following two tasks. First task: there is provided a yaw control technique for a wind turbine generator that can reduce the magnitude of the wind direction deviation even though the wind direction changes gradually over a long time while an increase in the number of yaw rotations is suppressed. Second task: there is provided a yaw control technique for a wind turbine generator that can improve the efficiency of the wind turbine generator by performing yaw rotations at an appropriate timing through early detection of transitional changes in the wind direction. In order to achieve the above-mentioned object, the invention includes means to be described below. Numbers and reference numerals used in [Best Mode for Carrying out the Invention] are given to the means in order to clarify a correspondence relationship between the description of [Claims] and the description of [Best Mode for Carrying out the Invention]. However, the given numbers and reference numerals are not used to limit the scope of the invention as described in [Claims]. A wind turbine generator includes a nacelle 3 in which a wind turbine rotor 7 is mounted, a rotating mechanism 4 that performs yaw rotation of the nacelle 3, a wind direction measuring means 10 that measures a wind direction, and a controller 21 that controls the rotating mechanism 4. The controller 21 calculates the wind direction deviation of the direction of the wind turbine rotor 7 and the wind direction measured by the wind direction measuring means 10, and performs yaw rotation of the nacelle by the rotating mechanism when any one of the following conditions (1) and (2) is satisfied. The conditions (1) and (2) include (1) a state where the absolute value of the wind direction deviation is not less than a first threshold (or a state where the absolute value of the wind direction deviation exceeds a first threshold) continues for a predetermined first duration, and (2) a state where the absolute value of the wind direction deviation is not less than a second threshold larger than the first threshold (or a state where the absolute value of the wind direction deviation exceeds a second threshold) continues for a second duration shorter than the first duration. Assuming that the current time is denoted by t0 and T denotes a predetermined value, yaw rotation of the nacelle 3 is stopped if an average of the wind direction between time t0-T and time t0 corresponds to the direction of the wind turbine rotor 7. The stop position of the yaw rotation is determined in this way by an average of the wind direction at a predetermined time T, so that it may be possible to stop yaw rotation of the nacelle 3 at an appropriate position, without stopping the yaw rotation by a momentary change in the wind direction. Accordingly, it may be possible to further reduce the wind direction deviation after stopping the yaw rotation than in the related art. Further, time elapsed until the next yaw rotation starts is lengthened by reducing the wind direction deviation after stopping the yaw rotation, so that it may be possible to decrease the number of yaw rotations. Furthermore, the number of the yaw rotations is suppressed and control performance is improved. Further, a wind turbine generator includes a nacelle 3 in which a wind turbine rotor 7 is mounted, a rotating mechanism 4 that performs yaw rotation of the nacelle 3, a wind direction measuring means 10 that measures a wind direction, and a controller 21 that controls the rotating mechanism 4. The controller 21 calculates the wind direction deviation from the direction of the wind turbine rotor 7 and the wind direction measured by the wind direction measuring means 10, and performs yaw rotation of the nacelle 3 by the rotating mechanism when any one of the following conditions (1) and (2) is satisfied. The conditions (1) and (2) includes (1) a state where the absolute value of the wind direction deviation is not less than a first threshold (or a state where the absolute value of the wind direction deviation exceeds a first threshold) continues for a predetermined first duration, and (2) a state where the absolute value of the wind direction deviation is not less than a second threshold larger than the first threshold (or a state where the absolute value of the wind direction deviation exceeds a second threshold) continues for a second duration shorter than the first duration. Yaw rotation of the nacelle 3 is performed by an angle which corresponds to the first or second threshold corresponding to at least one of conditions (1) and (2) satisfied at the time of the yaw rotation. In this way, yaw rotation of the nacelle 3 is performed by an angle, which corresponds to the first or second threshold corresponding to at least one of conditions (1) and (2) satisfied at the time of the yaw rotation. Accordingly, it may be possible to stop yaw rotation of the nacelle 3 at an appropriate position, without stopping the yaw rotation by a momentary change in the wind direction. Therefore, time elapsed until the next yaw rotation starts is lengthened by reducing the wind direction deviation after stopping the yaw rotation, so that it may be possible to suppress the number of yaw rotations. Furthermore, this control is effective when the degree of wind turbulence is particularly small. Moreover, the controller 21 rotates the nacelle 3 by an angle corresponding to the first or second threshold, determines whether a sign of the wind direction deviation becomes opposite to a sign of the wind direction deviation before the yaw rotation, stops the yaw rotation if the sign of the wind direction deviation becomes opposite to the sign of the wind direction deviation before the yaw rotation, and continues the yaw rotation until the wind direction deviation becomes zero if the sign of the wind direction deviation does not become opposite to the sign of the wind direction deviation before the yaw rotation. The controller rotates the nacelle by an angle corresponding to the first or second threshold, determines whether a sign of the difference between the direction of the wind turbine rotor and an average of the wind direction between time t0-T and time t0 becomes opposite to a sign of the difference before the yaw rotation assuming that the current time is denoted by t0 and T denotes a predetermined value, stops the yaw rotation if the sign of the difference becomes opposite to the sign of the deviation before the yaw rotation, and continues the yaw rotation until a difference between the average of the wind direction and the direction of the wind turbine rotor becomes zero if the sign of the wind direction deviation does not become opposite to the sign of the wind direction deviation before the yaw rotation. Accordingly, it may be possible to stop the yaw rotation at a more appropriate position. A wind turbine generator includes a nacelle 3 in which a wind turbine rotor 7 is mounted, a rotating mechanism 4 that performs yaw rotation of the nacelle 3, a wind direction measuring means 10 that measures a wind direction, and a controller 21 that controls the rotating mechanism 4. The controller (a) calculates the wind direction deviation from the direction of the wind turbine rotor 7 and the wind direction measured by the wind direction measuring means 10, (b) determines which situation of a first situation where the wind direction is changing randomly and a second situation where the wind direction changes gradually the current wind situation corresponds to, (c) determines that the current wind situation corresponds to the second situation, rotates the nacelle 3 by an angle corresponding to a first threshold when a state where the absolute value of the wind direction deviation is not less than a predetermined first threshold (or a state where the absolute value of the wind direction deviation exceeds the first threshold) continues for a predetermined first duration, determines whether a sign of the wind direction deviation becomes opposite to a sign of the wind direction deviation before the yaw rotation, stops the yaw rotation if the sign of the wind direction deviation becomes opposite to the sign of the wind direction deviation before the yaw rotation, and continues the yaw rotation until the wind direction deviation becomes zero if the sign of the wind direction deviation does not become opposite to the sign of the wind direction deviation before the yaw rotation, and (d) determines that the current wind situation corresponds to the first situation, and performs yaw rotation of the nacelle 3 so that an average of the wind direction between time t0-T and time t0 corresponds to the direction of the wind turbine rotor 7 assuming that the current time is denoted by t0 and T denotes a predetermined value, when a state where the absolute value of the wind direction deviation is not less than a second threshold larger than the first threshold (or a state where the absolute value of the wind direction deviation exceeds a second threshold) continues for a second duration shorter than the first duration. Accordingly, since it may be possible to perform the stop control of the yaw rotation in accordance with the current wind situation, control performance is improved. Further, a wind turbine generator includes a nacelle 3 in which a wind turbine rotor 7 is mounted, a rotating mechanism 4 that performs yaw rotation of the nacelle 3, a wind direction measuring means 10 that measures a wind direction, and a controller 21 that controls the rotating mechanism 4. The controller 21 calculates the wind direction deviation from the direction of the wind turbine rotor 7 and the wind direction measured by the wind direction measuring means 10, and performs yaw rotation of the nacelle 3 when the wind direction deviation satisfies a predetermined condition about all of time t satisfying "ts≤t≤t0" assuming that the current time is denoted by t0, T1 denotes a predetermined value, and time satisfying "t0-"t0-T1≤t≤t0" is denoted by ts. The predetermined condition is |Δθ(t)|≤ θTH( ts) . Meanwhile, |Δθ(t)| denotes the absolute value of the wind direction deviation at each time t, θTH(t) denotes a function that broadly uniformly increases in the range of "t0-T1≤t≤t0" and a derived function dθTH(t)/dt of θTH(t) with respect to time broadly uniformly increases in the range of "t0-T1≤t≤t0" except for time t where a derived function cannot be defined. When a function θTH (t)is prepared in this way and the predetermined condition is satisfied, the condition of the yaw rotation is flexibly set by performing yaw rotation of the nacelle 3. Accordingly, it may be possible to detect a transitional change in the wind direction early, and control performance is improved. Furthermore, a wind turbine generator includes a nacelle 3 in which a wind turbine rotor 7 is mounted, a rotating mechanism 4 that performs yaw rotation of the nacelle 3, a wind direction measuring means 10 that measures a wind direction, and a controller 21 that controls the rotating mechanism 4. The controller (a) calculates the wind direction deviation from the direction of the wind turbine rotor 7 and the wind direction measured by the wind direction measuring means 10, (b) determines which situation of a first situation where the wind direction changes at random and a second situation where the wind direction is gradually changed the current wind situation corresponds to, and (c) performs yaw rotation of the nacelle 3 when the wind direction deviation satisfies a predetermined condition about all of time t satisfying "ts≤t≤t0" assuming that the current time is denoted by t0, T1 denotes a predetermined value, and time satisfying nt0- "t0-T1≤t≤t0"is denoted by ts. The predetermined condition is |Δθ(t)|≤θTH1ts) if the current wind situation corresponds to the first situation. The predetermined condition is |Δθ(t)|≤θH2( ts) if the current wind situation corresponds to the second situation. Meanwhile, |Δθ(t)| denotes the absolute value of the wind direction deviation at each time t, θTH1(t) denotes a function that broadly uniformly increases in the range of "t0-T1≤t≤t0", and a derived function dθTH1(t)/dt of θTm(t) with respect to time broadly uniformly increases in the range of "t0-T1≤t≤t0" except for time t where a derived function cannot be defined, θTH2(t) denotes a function that broadly uniformly decreases in the range of "t0-T1≤t≤t0" and a derived function dθTH2(t)/dt of θTH2(t) with respect to time broadly uniformly increases in the range of "t0-"t0-T1≤t≤t0" except for time t where a derived function cannot be defined. Accordingly, since it may be possible to perform the stop control of the yaw rotation in accordance with the current wind situation, control performance is improved. In addition, assuming that the current time is denoted by t0 and T denotes a predetermined value, the controller stops the yaw rotation of the nacelle if an average of the wind direction between time t0-T and time t0 corresponds to the direction of the wind turbine rotor. Further, the controller rotates the nacelle by an angle corresponding to θTH1ts) or θTH2(ts), determines whether a sign of the wind direction deviation becomes opposite to a sign of the wind direction deviation before yaw rotation, stops the yaw rotation if the sign of the wind direction deviation becomes opposite to the sign of the wind direction deviation before the yaw rotation, and continues the yaw rotation until the wind direction deviation becomes zero if the sign of the wind direction deviation does not become opposite to the sign of the wind direction deviation before the yaw rotation. Furthermore, the controller measures the number of yaw rotations that is performed between the current time and a predetermined time, eases the rotation conditions if the number of yaw rotations is smaller than a predetermined number of rotations, and tightens the rotation conditions if the number of rotations is larger than a predetermined number of rotations. Accordingly, the upper limit of a predetermined frequency of yaw rotations is maintained, and performance of the control for reducing the wind direction deviation is improved. According to the invention, it may be possible to improve the efficiency of the wind turbine generator. More specifically, according to the invention, it may be possible to provide a yaw control technique for a wind turbine generator that can reduce the magnitude of the wind direction deviation even though the wind direction changes gradually over a long time and can suppress an increase in the number of yaw rotations. Further, according to another embodiment of the invention, it may be possible to improve the efficiency of the wind turbine generator by performing yaw rotations at an appropriate timing through the early detection of transitional changes in the wind direction. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flowchart illustrating a procedure of yaw rotation control that is performed by a controller of Example 1. Fig. 2 is a view comparing the yaw control, which is performed by control logics in the related art and of intermediate embodiments during the stopping of yaw rotation, with the yaw control that is performed by a control logic of Example 1. Fig. 3 is a flowchart illustrating a procedure of yaw rotation control that is performed by the controller of Example 2. Fig. 4 is a view comparing the yaw control, which is performed by a control logic during the stopping of yaw rotation in the related art and an intermediate embodiment, with the yaw control that is performed by a control logic of Example 2. Fig. 5 is a flowchart illustrating a procedure of yaw rotation control that is performed by a controller of Example 3. Fig. 6 is a view comparing the yaw control, which is performed by a control logic during the stopping of yaw rotation in the related art and an intermediate embodiment, with the yaw control that is performed by a control logic of Example 3. Fig. 7 shows graphs showing control logics of Examples 1 to 4, Fig. 7(a) shows graphs showing control logics used in Examples 1 to 3, and Figs. 7(b) and 7(c) show graphs showing a control logic of Example 4. Fig. 8 is a flowchart illustrating the procedure of yaw rotation control of the invention. Fig. 9 is a view showing the configuration of a wind turbine generator according to an embodiment of the invention. Fig. 10 is a cross-sectional view showing the configuration of a nacelle rotating mechanism of the embodiment of the invention. Fig. 11 is a block diagram showing the configuration of a yaw control system of a first embodiment of the invention. Fig. 12 shows graphs showing the change of the wind direction deviation that is caused by a control logic in the related art, and the change of the wind direction deviation that is caused by a control logic of a first intermediate embodiment. Fig. 13 shows graphs showing an example of the determination of "a situation where the wind direction changes at random so that the degree of turbulence is large" and "a situation where the wind direction changes gradually over a long time". Fig. 14 shows graphs showing the change of the orientation of the wind turbine that is caused by a control logic of a second intermediate embodiment, and a change in the orientation of the wind turbine that is caused by the control logic in the related art. Fig. 15 is a graph showing the efficiency of a wind turbine generator that is obtained by the control logic of the intermediate embodiment and the control logic in the related art in a case where the rate of change in the wind direction is constant and the width of change of the wind direction deviation is the same. Fig. 16 shows graphs showing the start timing of the yaw rotation in the control logic in the related art and the start timing of the yaw rotation in a control logic of a third intermediate embodiment. Fig. 17 is a graph showing an example of a function eTH(t). Fig. 18 is a graph showing the control logic in the related art. Fig. 19 shows graphs illustrating problems of the control logic in the related art. DETAILED DESCRIPTION OF THE INVENTION Preferred examples of the invention will be described in detail below with reference to drawings. However, unless being particularly described, the dimensions, materials, and shape of components described in these examples, and the relative disposition thereof do not limit the scope of the invention and are merely illustrative. Fig. 9 is a side view showing the configuration of a wind turbine generator 1 according to the invention. The wind turbine generator 1 includes a tower 2 and a nacelle 3 that is provided at the upper end of the tower 2. The nacelle 3 is rotatable in a yaw direction, and is directed to a desired direction by a nacelle rotating mechanism 4. The nacelle 3 is provided with a wound-rotor induction generator 5 and a gear 6. A rotor of the wound-rotor induction generator 5 is coupled with a rotation shaft 7a of a wind turbine rotor 7 through a gear 6. The wind turbine rotor 7 includes a hub 8 that is connected to the rotation shaft 7a, and blades 9 that are fixed to the hub 8. The nacelle 3 is further provided with a wind vane/anemometer 10 that measures wind speed and a wind direction. Fig. 10 is a cross-sectional view showing an example of the configuration of the nacelle rotating mechanism 4. The nacelle rotating mechanism 4 includes a yaw motor 11, a decelerator 12, a pinion 13, an internal gear 14, a yaw braking mechanism 15, and a brake disc 16. The yaw motor 11, the decelerator 12, the pinion 13, and the yaw braking mechanism 15 are mounted on the nacelle 3, and move together with the nacelle 3. Meanwhile, the internal gear 14 and the brake disc 16 are fixed to the tower 2. A rotor of the yaw motor 11 is mechanically connected to the pinion 13 through the decelerator 12, and the pinion 13 and the internal gear 14 are engaged with each other. When current is supplied to the yaw motor 11, the pinion 13 rotates, so that yaw rotation of the nacelle 3 is performed. Yaw rotation of the nacelle 3 is braked by the braking mechanism 15. When brake shoes 17 of the yaw braking mechanism 15 clamp the brake disc 16, yaw rotation of the nacelle 3 is braked or stopped. Fig. 11 is a block diagram showing an example of the configuration of a control system for yaw control. In this embodiment, a yaw control system includes a controller 21, a motor drive unit 22, and a braking mechanism drive unit 23. The motor drive unit 22 supplies driving power to the yaw motor 11 in accordance with a control signal sent from the controller 21. The braking mechanism drive unit 23 makes the brake shoes 17 of the braking mechanism 15 clamp the brake disc 16 in accordance with a control signal sent from the controller 21. The controller 21 decides a desired direction of the wind turbine rotor 7 from the wind speed and the wind direction that are measured by the wind vane/anemometer 10, and performs yaw rotation of the nacelle 3 by operating the yaw motor 11 so that the wind turbine rotor 7 is directed to a desired direction. Further, if the wind turbine rotor 7 is directed to a desired direction by the yaw rotation, the controller 21 brakes the yaw rotation by operating the braking mechanism 15. (First intermediate embodiment) The yaw control according to a first intermediate embodiment, which is obtained by modifying the yaw control of the wind turbine generator in the related art having been described with reference to Fig. 18, will be described below. In the first intermediate embodiment, the yaw control is performed in response to the wind direction that is measured by the wind vane/anemometer 10. More specifically, the controller 21 performs the yaw control as follows: The wind vane/anemometer 10 measures a wind direction, which corresponds to each time, at predetermined sampling intervals, and supplies wind direction data, which represent the wind direction corresponding to each time, to the controller 21. In the wind direction data, the wind direction is defined as an angle that is formed by a predetermined reference direction. The controller 21 generates wind direction data for control, which are actually used for yaw control, by performing a low-pass filtering of the measured wind direction data (most simply, by averaging several wind direction data that are temporally adjacent), and calculates the difference between the orientation of the wind turbine and the wind direction, which is represented in the wind direction data for control, as a wind direction deviation. In the first intermediate embodiment, the orientation of the wind turbine is represented as an angle that is formed between a predetermined reference direction and the direction of the rotation shaft 7a of the wind turbine rotor 7. The wind direction deviation is a datum that has one of a positive value, a negative value, or zero. The value, which is obtained by subtracting the angle of the orientation of the wind turbine from the wind direction represented in the wind direction data for control, is defined as a wind direction deviation in the first intermediate embodiment and embodiments to be described below. Further, the controller 21 performs yaw rotation of the nacelle 3 by controlling the motor drive unit 22 and the braking mechanism drive unit 23 in response to the calculated wind direction deviation. If satisfying at least one of the following two conditions in the first intermediate embodiment, the controller 21 performs yaw rotation so that the wind direction deviation becomes zero (that is, in a direction that is indicated by the newest wind direction data for control). (1) A state where the absolute value of the wind direction deviation is not less than a threshold θTH1 (or exceeds θTH1) continues for Tx seconds. (2) A state where the absolute value of the wind direction eviation is not less than a threshold ΘTH2(>ΘTH1) (or exceeds θTH2) continues for T2(θTH1) (or exceeds θTH2) continues for T2 (for example, 20 seconds)(