An electric power steering apparatus being provided with: a torque sensor 10 for detecting a steering torque applied to a steering wheel (not shown); and a steering assisting motor 24 driven by a drive circuit 13 to which the voltage of a mounted battery P is applied, in which the drive circuit 13 rotates the motor 24 in accordance with a motor current instruction value determined based on the steering torque detected by the torque sensor 10, is further provided with: judging means 12 for judging whether the motor current instruction value is higher than a first threshold value or not; boosting instruction outputting means 12 for, when the judging means 12 judges that the motor current instruction value is higher, outputting a boosting instruction for boosting the voltage of the mounted battery P; and a boosting circuit 8d for boosting the voltage of the mounted battery in response to the outputted boosting instruction. Even when the voltage of the mounted battery P fluctuates, a voltage from which a stable steering assisting force is always obtained can be supplied to the steering assisting motor, so that the steering assisting motor can be reduced in size.