The invention provides a chain tensioner which can suppress the change of damping characteristics along with the change of oil temperature and the increase of the manufacturing cost through the automatic adjustment of the flow channel resistance in an oil leakage channel with a simple structure. The present invention provides the chain tensioner (100) that automatically adjusts flow path resistance inside an oil leak path (115) with a simple configuration to make it possible to reduce a change in damping characteristics resulting from a change in oil temperature and reduce an increase in manufacturing cost. In the chain tensioner (100), a pressure oil chamber is provided with an oil leak path (115) communicating with an outside of the chain tensioner (100), the oil leak path (115) containsa movable piece (133) which is movable with a temperature-dependent deformation member (131) that deforms according to oil temperature, and the movable piece is formed into a shape that changes flowpath resistance of the oil leak path according to a position to which the movable piece is moved.