An assisting power controller for a vehicle power steering device is provided with a microcomputer, which stores in a random access memory a plurality of the latest steering angle data each detected from a steering angle detector each time the vehicle runs a predetermined distance. The microprocessor processes the plurality of steering angle data to calculate a first driving index representing the road condition and then, to decide a first electric current meeting the road condition. The microprocessor also stores in the memory a plurality of the latest vehicle speed data each detected from a speed sensor each time the vehicle runs the predetermined distance. The microprocessor processes the plurality of vehicle speed data to calculate a second driving index representing the state of maneuver by the driver of the vehicle and then, to decide a second electric current meeting the state of maneuvering. The microprocessor thereafter makes an addition of the first and second electric currents and applies the added electric current to an electromagnetic flow control valve which controls the bypass communication of pressurized fluid between opposite chambers of a power cylinder, to control the assisting power generated by the power cylinder based on the road condition and the state of maneuvering by the driver of the vehicle.