To provide a chain tensioner that can maintain the pressure of the oil pressure chamber at a correct level to achieve a desired reaction force and damping characteristics corresponding to the rpm, and enable a reduction in the number of parts to be machined and the number of assembling steps; reduce the amount of oil flowing out of the system; and prevent damage to the check valve, without causing an increase in size of the chain tensioner. The chain tensioner includes a check valve unit 150 that stops reverse flow of oil flowing into an oil pressure chamber 101, and a first relief valve unit 160 and a second relief valve unit 170 that release oil when the pressure reaches or exceeds a predetermined high level. The second relief valve unit 170 has a higher valve opening pressure than that of the first relief valve unit 160.