A power-assisted steering system includes a current detector for detecting sequentially current flowing to a ground-side switching element in a lower arm by a detection resistor inserted into the lower arm not only in a time period when the switching element is ON but also in time period when the switching element is OFF, an offset updating unit for sequentially updating an offset correction value with detected values in the time period when the switching element is OFF, and a detected current correcting unit for calculating a motor current value (a corrected current detected value) by correcting detected values in the time period when the switching element is ON with the offset correction value thereby updating.