A power transmission unit for preventing malfunction and deterioration in durability of a locking device arranged in a housing by preventing the ingress of foreign matters into the locking device. In the power transmission unit, a one-way brake 10 for selectively stopping a rotation of an output shaft 1a of an engine 1 is disposed between the housing 11 of a transmission mechanism and an engine block 1b. A fixed member 10a is fixed to the housing 11 or to the engine block 1b, and a movable member 10b is attached to the output shaft 1a. A through hole 21 is formed on the housing at a vertically lowest portion and axially outside of a region where the one-way brake 10 is situated, so as to provide a communication between an inner space and an outer space of the housing 11.