To provide a chain tensioner that has a simple structure, is universally usable, is capable of supplying sufficient and necessary oil when restarted after a long time after the stop of oil supply, enables reduction in entire length and size, and enables easy production and reduction in production cost. An urging unit 140 provided in an oil pressure chamber 101 to be expandable and contractible urges a plunger 120 outward from a plunger bore 111 in a tensioner body 110 of the chain tensioner 100. The plunger 120 includes a plunger body 126, a check valve 150 provided inside the body, and a reserve tube 130 that has an oil reservoir chamber 123 communicating with the oil pressure chamber 101 through an oil hole 132.