A hydraulic chain tensioner (1) comprises a hollow plunger (3) slidably received within the bore (2) of a housing (3). A spring (4) pushes the plunger (3) in the protruding direction, and a pressure relief valve assembly (5) and an air vent disc (10) that are integrally incorporated on the upper end at the tip of the plunger (3). The air vent disc (10) has a disc shape including a first and second face. An outer periphery is formed with a trough-hole (31A). The outer diameter of the air vent disc (10) is slightly smaller than the inner diameter of the plunger (3). The first face is formed with a spiral groove and the circumferential edge portion of the first face is formed with a radius. The starting end of groove is disposed at the radius and the terminal end of the groove is centrally located on the first face.