A hydraulic auto-tensioner includes a high-pressure oil chamber and a low-pressure oil chamber, and oil (L) in the high-pressure oil chamber passes through a clearance (G) between a cylinder (17) and a piston portion (18) of a plunger and leaks into the low-pressure oil chamber from an end of the clearance (G) near an end surface of the cylinder (17). A recessed groove (40) is provided in an inner peripheral surface of the cylinder (17) by a conical surface (41) having a groove angle of 5 to 15 degrees from the end of the clearance (G), a throttle portion (45) projecting onto the piston portion (18) side is provided further toward the end surface side of the cylinder than the groove (40) in the inner peripheral surface of the cylinder (17), and a throttle gap (A) larger than the clearance(G) is formed between the throttle portion (45) and the piston portion (18).