PURPOSE: To enable to keep the engine speed at a desired speed even if some trouble is caused with an air lock mechanism, by providing a lock piston and grooves in a plunger which is moved up and down by the depressive force acted to a pedal of an accelerator valve having an air lock mechanism operated by fluid pressure.

CONSTITUTION: An accelerator pedal 47 is interconnected to a stepped piston 15 of an accelerator valve 2 via a plunger 43. The valve body 2 has an air lock mechanism consisting of an air supply port 9, an air outlet port 11, an exhaust port 12, an air supply and discharge port 27, etc. A plurality of circumferential grooves 45 are formed at a portion of the outer surface of the plunger 43, and a lock means having a lock piston 57 is provided at the right half of a cap member 41. When a driver depresses the pedal 47 and pushes a lock command button, compressed air is supplied to an inpt port and the lock piston 57 is moved to the left, so that a pawl 62 at the top of the lock piston 57 is engaged with a mating groove 45. Therefore, the accelerator pedal 47 is fixed at the position even if the driver's foot is detached from the pedal 47 and air is leaked from the air lock mechanism.