<p>A chain tensioner (1) of low production cost. A plunger (14) is slidably inserted in a cylinder (11) to form a pressure chamber (15) in the cylinder (11). An oil supply path (16) is provided in the cylinder (11), and a check valve (17) is provided at that end of the oil supply path (16) which is on the pressure chamber (15) side. A leakage clearance (18) is formed between sliding surfaces of the plunger (14) and the cylinder (11). A return spring (28) is installed in the pressure chamber (15), the plunger (14) is urged by the return spring (28) in the direction of projecting from the cylinder (11), and thus a chain (6) is pressed by the plunger (14). The plunger (14) has a through hole (31) extending up to the pressure chamber (15) from that end surface of the plunger (14) which projects from the cylinder (11). A plug (32) having a female screw thread (33) on its outer periphery is press fitted in the through hole (31) to form a helical air bleeding path (34) between the inner periphery of the through hole (31) and the outer periphery of the plug (32).</p>
