Assist force that assists an operation of a steering system of a vehicle is applied to a rack shaft by transmitting driving force of a motor to the rack shaft via a motor shaft and a ball screw nut. The high-frequency power amplifier has at least one stage of an amplifier element. The housing unit is rotatable about an axis substantially perpendicular to a bottom surface thereof. A first setpoint temperature and a second setpoint temperature are selected. The louver module of includes a frame, an openingclosing mechanism, a louver unit and power transmission mechanism. Citric acid may optionally be present.