To surely detect a back electromotive force generated in a non-conduction phase at an extremely low duty ratio, a motor driving system includes a three-phase motor, an inverter circuit, and a semiconductor device. A controller included in the semiconductor device compares a voltage at an output node corresponding to a non-conduction phase of the inverter circuit and a reference voltage with each other, thereby estimating a position of a rotor of the three-phase motor and generating a pulse width modulation signal based on the estimated position of the rotor. The controller detects the voltage at the output node of the non-conduction phase in a regeneration period of the pulse width modulation signal when a duty ratio of the pulse width modulation signal is less than a threshold value, the regeneration period being a period in which current is made to flow to the three-phase motor on a regeneration path.