To enable the assembling accuracy of a reduction roller type reduction gear to be set to a relatively lower level to improve productivity in an assist power transmission system for power assist vehicle in which the reduction roller type reduction gear is arranged between an electric motor and a rotary driving shaft giving rotational power to drive wheels.

First- and second casing half-bodies 76, 77 form a storage chamber 79 between themselves to be connected to each other, and a motor shaft 84 of an electric motor 34 installed on the outer surface side of the first casing half-body 76 is rotatably supported on a first bearing part 89 provided at the first casing half-body 76. Then, each reduction roller 87 etc., are rotatably supported in the first casing half-body 76, and the tip part of an output shaft 115, the base end part of which is locked to the closed end-center part of a bowl-like outer ring 85, is rotatably supported on a second bearing part 91 provided in the second casing half-body 77. A power transmission section 107 is provided on the output shaft 115 between the outer ring 85 and the second bearing part 91.