A power steering device for use with a boat propulsion unit has a hydraulic cylinder unit for angularly moving a steering arm of the boat propulsion unit. A rotary valve mechanism for supplying working oil from a working oil tank to the hydraulic cylinder unit and discharging working oil from the hydraulic cylinder unit to the working oil tank comprises a flat valve body having oil holes defined transversely therethrough and a valve housing assembly housing the flat valve body and having ports defined therein for communication with the oil holes, the ports being connected to the working oil tank and the hydraulic cylinder unit. The power steering device also includes a steering shaft for operating the rotary valve mechanism in response to a steering action. The hydraulic cylinder unit, the rotary valve mechanism, and the steering shaft are integrally joined to each other.