To provide a power transmission device which can exert good braking effect, has good durability, can prevent deformation and breakage of members, achieve a compact and lightweight construction, and ensures smooth operations even at the time of reverse operation.

A power transmission device is composed of a pair of levers 21 and 22 installed swingably around a support point 19 on a disc 18 where a brake mechanism 12 is coupled with an output shaft 11, a C-form resilient break piece 23 to be in slide contact with the inside surface of a casing 9 in such a way as capable of being mounted removably, and a rotational driving part 24 coupled with the input shaft 10. The disc 18 is furnished with a cutout groove 25 at the periphery and a pair of bosses 26 and 27 rising on the oversurface, and the levers 21 and 22 are located between the bosses 26 and 27, and the inside circumference (a) of their forefront parts are lapped inward to a minor extent from the two ends (b) of the cutout groove while the periphery of the base end is abutted to the end of the brake piece 23. The rotational driving part 24 has a driving piece 24a rising at the end, and this piece 24a is inserted into the cutout groove 25.