To provide a belt tension mechanism for regulating the excessive displacement of an engine without providing any stopper.

In the belt tension mechanism provided with a drive side pulley 1 attached to an engine, a driven side pulley 2, a belt 3 laid between the pulleys, and a tension pulley 4, the tension pulley 4 is connected to a rotatable tension arm 5, the tension arm 5 is so arranged that tension load acts on a straight line linking the rotational fulcrum of the tension arm 5 and the center of the tension pulley 4 when the engine is displaced, and the displacement of the engine is regulated by belt tension.