PURPOSE: To ensure the safety of a system by providing a stopper in a part of a controlled variable input system, which can be on-off operated from the outside in such a manner that the operation range of a servo spool is regulated to keep an output member from entering one operation region.

CONSTITUTION: A servo spool 6 which is a constitutional element of the terminal end of a controlled variable input system A is slidably fitted in a spool holding hole 1a of a pintle 1, and a stopper 10 which can be on-off operated to be projected and recessed by an external valve 11 in such a manner as to face on the rear end 6b thereof. The stopper 10 comprises a stopper cylinder 10a fixed to a housing 3 and a stopper piston 10b, wherein only when it is held in a projecting position of the stopper piston 10b, the servo spool 6 is kept from going over the boundary position to enter the negative region. Thus, the pintle 1 can be prevented from going over the boundary position due to an operational error of an operator so as to ensure the safety of a system.