To provide a tensioner that enables, with a simple structure, reduction of installation space of the tensioner and reduction of production costs. The tensioner includes a plunger, a housing, a spring, and a locking mechanism that stops the plunger from projecting out to the front side. The locking mechanism includes a first locking member and a second locking member. The first locking member includes a first spring holder arranged on a first side of a first end of the spring, and a restrained part. The second locking member includes a second spring holder arranged on a second side of a second end of the spring, and a movement restricting part that restricts movement of the restrained part toward the first side. The restrained part and movement restricting part are configured to releasably engage with each other.