The present invention is a control apparatus for controlling an electric power steering system having steering assist force control means for controlling a motor that gives a steering assist force to a steering mechanism relying upon a main torque input value obtained by subjecting a main torque signal from a torque sensor to the A/D conversion, upon a gain-multiplied torque input value obtained by multiplying said main torque signal by a first gain times followed by the A/D conversion and upon a sub-torque signal from said torque sensor, in order to carry out a failsafe function, characterized by the provision of a gain multiplier for multiplying said main torque input value by a second gain times,. and switching means for changing the output of said gain multiplier and said gain-multiplied torque input value over to each other to input it to said steering assist force control means.