A fail-safe system is used in a control unit for opening and closing guide vanes of a pump-turbine apparatus operable in both of pump operation mode and turbine operation mode. The fail-safe system has a wire carried by a rotatable sheave adapted for transmitting an information concerning an opening degree of the guide vanes, a weight suspended from one end of the wire, an operation rod disposed below and in alignment with the weight in the direction of action of the force of gravity, and a change-over valve adapted to be switched by the operation rod. When the wire is cut off accidentally, the weight drops onto the operation rod to depress the latter to switch the change-over valve so that a servo-motor is switched to make the guide vanes close at predetermined rates of change of the opening degree in respective operation modes.