To enable execution of a continuous positional control, to make a shaft stop surely without overrunning a target point and to improve the sphere of usage and the reliability of a fluid pressure actuator.

A magnet 10 for position detection which constitutes a position detecting device is provided on an auxiliary shaft 5 which is engaged with a fitting ring 7 provided perpendicularly to the axis of a shaft 3 and is disposed outside 6 a cylinder case 2 and in parallel thereto. Since the auxiliary shaft 5 reciprocates in linkage with the shaft 3, the magnet 10 for position detection moves with the movement of the shaft 3 and a plurality of magnetic poles 23, 24, 25 and 26 are detected by a first magnetic sensor 17 and a second magnetic sensor 18. As the result, the shaft 3 can be decelerated on this side of a first point and a second point.