PURPOSE: To provide a fluid pressure chuck having a position detecting mechanism which is not influenced by work environment.

CONSTITUTION: A device is composed of a movable body 24 in which a position detection mechanism 5 to detect a position of a finger 4b is incorporated between a body 7b of an oscillation type actuator 1 and a guide holder 12, an object body 31 to be detected provided on the movable body 24, and a position detecting sensor 33 movably fitted on switch fitting grooves 32, 32 in the body 7b to generate a signal by motion of the object body 31 to/from it. The movable body 24 which is moved in the same direction as the finger 4b is provided with a base part 24a installed on a finger pin 23a of the finger 4b, an attitude holding part 24b to move across a shaft 2 of the oscillation type actuator 1, and an fitting part 24c having the object body 31.