In an electric power steering system, during initial processing after an ignition switch is turned on, when a main CPU completes calculation of an offset voltage, it sends an offset voltage calculation request signal to a sub CPU. When the time from sending the offset voltage calculation request signal until the main CPU receives an offset voltage calculation completion signal from the sub CPU is within a predetermined period of time, it is determined that a communication function between the main CPU and the sub CPU and an offset voltage calculation function of the sub CPU are normal, and at the same time it is possible to maintain synchronism of the offset voltage calculation in the main CPU with the offset voltage calculation in the sub CPU. Thus, it is possible to carry out in a short time a check of the communication function between the main CPU and the sub CPU, while maintaining the synchronism in the offset voltage calculations by the two CPUs when calculating an actual motor current.