To regulate the advance end position of a rod without forming a large diameter part for a stopper on the outer peripheral surface of a rod, in a direct acting actuator comprising a hollow rod whirl-stopped at a cylindrical casing and axially movably inserted and supported screw shaft loosely inserted in the rod, and a nut joined in a screwed-in state with the screw shaft mounted on the tail end part of the rod, and having the rod moved forward and backward through normal and reverse rotation of the screw shaft.

A stopper ring 14 is mounted on the outer periphery of the tip part of a screw shaft 8 and the forward end position of a rod 4 is regulated through contact of a nut 10 with the stopper ring 14. The outside diameter of the stopper ring 14 is adjusted to a value approximately equal to the inside diameter of the rod 4 and oscillation of the screw shaft 8 is prevented from occurring. In this case, the stopper ring 14 is rotatably inserted externally of the screw shaft 8 and thrust exerted on the stopper ring 14 is received by a thrust bearing 15 mounted on the screw shaft 8.