PURPOSE: To provide a tensioner capable of effectively preventing any jump-out of a plunger in assembling and excellent in protectability for a chain or the like.

CONSTITUTION: A tensioner is provided with a plunger 2 inserted into an oil chamber 10, a ratchet part 3 regulating any reverse of this plunger 2, and a jump-out regulating part 4 which is, in turn, provided with a piston chamber 40 formed in a sense almost orthogonal with a axis of the oil chamber 10, a lock piston 42 inserted into this piston chamber 40 and an energizing spring 44 energizing this lock piston 42. When the tensioner is assembled in a cylinder head, an engaged projection 42d is engaged with an engaged hole 26 by dint of this energizing spring 44. When hydraulic pressure works on the oil chamber 10, hydraulic pressure in this oil chamber 10 works on two pressure receiving surfaces 42a and 42c of the lock piston 42, whereby this piston 42 shifts in direction of an arrow Y6. If so, the engaged projection 42d separates from the engaged hole 26 of the plunger 2, so that a lock by the lock piston 42 is thus released.