A motor drive device has a spindle motor driver adapted to receive electric power from a power supply line to drive a spindle motor; an isolation switch adapted to connect and disconnect an application terminal of a power supply voltage to and from the power supply line, and a current monitor adapted to monitor a first current flowing into the isolation switch. The current monitor includes a mirror switch arranged to connect a first terminal of the mirror switch to the application terminal of the power supply voltage and adapted to be turned on and off by the same control signal as the isolation switch; a bias generator adapted to bias a second terminal of the mirror switch to the same voltage as the power supply line; a resistor which converts a second current flowing into the mirror switch into a first voltage; and a current limit signal generator adapted to compare at least one of the first voltage, a second voltage obtained by level-shifting the first voltage, and a third voltage obtained by dividing the first voltage with a predetermined threshold voltage to generate a current limit signal.