Provided is a simple-structured tensioner that can reduce processing and assembling workload and maintain the tensioner performance constant, as well as store a sufficient amount of oil. The tensioner includes a plunger 20, a housing 30, an inner sleeve 40, a check valve 50, and a biasing unit 60. The inner sleeve 40 includes a cylindrical body 41 having an inner part that serves as an oil reservoir chamber 13, a sleeve bottom 42 formed at one end of the cylindrical body 41, an oil hole 43 formed in the sleeve bottom 42, and a ball seat 44 which protrudes in a cylindrical shape toward an oil pressure chamber 12 integrally with and continuously from an edge of the oil hole 43 and on which a check ball 51 is seated.