A power steering system of an all-terrain buggy vehicle is provided to protect a power assist motor against obstacles encountered during traveling on the irregular ground, or the like. In a power steering system of a four-wheeled buggy vehicle in which a steering shaft rotated in response to the operation of a handlebar is disposed forward of an engine located at the almost-middle in the back-and-forth direction of a body frame, and the rotary shaft of a power assist motor is disposed perpendicular to the steering shaft, the power assist motor is located inside the body frame.