A power transmission device for vehicles comprises: a housing rotatably supporting a crank shaft of an engine of the vehicle; a first shaft rotatably supported inside the housing to be coaxial with the crank shaft; a clutch unit having a drive member arranged inside the housing and coupled to the crank shaft, and a driven member fixed to the first shaft; a second shaft rotatably supported inside the housing to be parallel to the first shaft. The device has also infinite variable transmission including a drive pulley consisting of a fixed sheave and an axially movable sheave mounted on the first shaft, a driven pulley consisting of a fixed pulley and an axially movable sheave mounted on the second shaft, and a V belt which is placed the driving and driven pulleys through V grooves defined by the fixed and axially movable sheaves of the driving and driven pulleys. Hydraulic piston-cylinder assemblies are fixed to the axially movable sheaves and to the associated first and second shafts so that they move the axially movable sheaves towards and away from the associated fixed sheaves. A rotary pump is disposed at the housing has a rotary element operatively coupled to the crank shaft for rotation therewith to generate a hydraulic pressure upon rotation, and which supplies the hydraulic pressure to the hydraulic piston-cylinder assemblies.