1 and a shoulder of the plunger 2 a piston 9 is inserted in a movable manner in a bore 8 bored in the center of the plunger 2 in the bottom of the bore 8 is formed a small diameter oil chamber 11 the chamber 11 being in communication with the large diameter oil chamber 4 by means of small diameter hole 10 the small diameter oil chamber 11 is sealed by means of an annular seal member 12 and cannot be communicated with the outside a snap ring 13 is attached to an end opening of the bore 8 the snap ring 13 can come into contact with a step 14 of the piston 9 and this contact prevents the piston 9 from moving entirely outside of the bore 8 a head 9a of the piston 9 extends outside body member 1 the end face of 9b and 9a defining a press face of the hydraulic autotensioner when using the device described above as a hydraulic autotensioner for a belt the head end face 9b is pressed against the belt face either directly or indirectly by means of another member the pressing force of the autotensioner against the.