To provide a highly reliable hydraulic auto-tensioner which prevents over a long time occurrence of inconveniences such as function degradation and abnormal noise due to cavitation, aeration, check delay, or the like, suppresses adhesive abrasion between a check ball and an annular ball seat, and reduces adsorption, without using expensive material.

The hydraulic auto-tensioner 10 has a check valve 28 allowing communication between a high-pressure oil chamber 26 and a low-pressure oil chamber 27 in one direction from the low-pressure oil chamber 27 to the high-pressure oil chamber 26. Film 4 formed by TiN coating, TiCN coating, DLC coating, or electroless nickel deposition is disposed one or both of a surface of the annular ball seat 32 for receiving the check ball and a surface of the check ball 30 of the check valve 28.