An unlocking controller is provided for an irreversible rotary transmission system having the irreversible rotary transmission system having an irreversible rotation transmission element arranged between an input shaft and an output shaft. The unlocking controller includes an input shaft rotation direction determination section and an unlocking torque setting section. The input shaft rotation direction determination section determines whether an input shaft rotational direction is the same as, or opposite to, a direction of the load torque of the output shaft. The unlocking torque setting section conducts an unlocking torque control that sets the unlocking torque a higher value when the input shaft rotational direction and the direction of the load torque of the output shaft are the same as while the lock is released, than when the input shaft rotational direction is opposite to the direction of the direction of the load torque of the output shaft.