To reduce vibration of a hydraulic drive device caused by fluctuation of control hydraulic pressure.

When executing an opening/closing test in a main steam stop valve 14, a closing signal is given to a solenoid valve 40 to operate the solenoid valve 40 such that a control oil discharged from a control oil drain outlet 101 is shut off. Since the hydraulic pressure in a space on the drain side of spaces inside a relay cylinder 104 partitioned by a relay piston 102 becomes high, the relay piston 102 moves in the direction X shown in Figure. Because a drive oil discharge passage 112 and a drive oil passage 113 are underlapped in this state, the control oil inside a power cylinder 111 is discharged from a drive oil drain 114. Further, since the main steam stop valve 14 is installed with a return spring 14r operating such that the return spring 14r closes the main steam stop valve 14, the drive oil inside the power cylinder 111 is rapidly discharged. Thereby, the main steam stop valve 14 is rapidly closed.