PURPOSE: To enhance the safety of an actuating device during operation by providing a lock mechanism in a fluid valve so that a protecting cover cannot be removed when the operating part of the actuating device is driven.

CONSTITUTION: A fluid valve has a control valve for opening and shutting off a fluid passage in a valve body 32, 'and the control valve is manipulated by a control part 31 so as to change over the opening and shutting off of the fluid passage. The control part 31 can be attached to and removed from the valve body 32 by means of a lock mechanism composed of a protrusion 31a of the control part 31 and a groove 32 in the valve body 32b when the fluid passage is shut-off, but the control part 31 cannot be removed when the fluid passage is opened. If the control part 31 is attached to a protecting cover for an actuating device, the protecting cover cannot be removed during operation of the actuating device.