A driving control unit (21b) of a controller (21) determines which of an input shaft (32) and an output shaft (33) in a steering shaft (3) is dynamically superior and when the output shaft (33) is dynamically superior to the input shaft (32), a motor (6) is driven so that torque resulting in a time integral value smaller than a time integral value of relative torque between the input shaft (32) and the output shaft (33) is generated in a backward assist direction.