Abstract: To provide a simple-structured chain tensioner that reduces the burden of work required for assembling and maintenance, sufficiently damps the motion of the plunger, and helps reduce machining costs and size increases of the plunger and tensioner body. The chain tensioner includes a tensioner body with a plunger bore, a plunger, and a biasing member that biases the plunger. The plunger has raised portions with which a locking member engages. The plunger bore is provided with a locking member holding part on one side that is open. The locking member holding part is provided with an inclined surface at one end in the plunger entry direction, the inclined surface becoming progressively shallower in the plunger entry direction. The locking member can deform elastically in a cross section when engaging with the raised portion of the plunger.