A motor driving device includes an analog to digital conversion circuit to which a speed command voltage instructing a rotation speed of a motor is inputted, and a microcomputer configured and programmed to generate a speed command having a pulse width which is changed according to the speed command voltage based on an output signal of the analog to digital conversion circuit. Further, the motor driving device includes a signal isolator configured to transmit the speed command between an input and an output while providing isolation between the input and the output, and a drive circuit configured to generate a driving signal in response to the speed command outputted from the signal isolator and to change the rotation speed of the motor.