To provide a power transmission device that has a structure employing a small number of components and easily adjustable in tension in view of problems especially of a device using belt transmission whose transmission efficiency is affected by tension.

The power transmission device comprises a structure where pulleys are arranged on a driving side and a follower side and a belt is entrained about them, and the pulleys and the belt are arranged for a plurality of stages to permit a speed change into a plurality of stages. A tension member 8 is disposed on reaches 6A and 7A of the plurality of belts 6 and 7 on the same side and between the outer surface of one belt and the inner surface of the other so as to move relatively to and away from each belt 6, 7. The contact of the tension member 8 with either of the plurality of belts 6 and 7 can increase the tension of the belt that is in contact with it to transmit torque to the pulleys selectively.