A motor drive device determines whether or not a combination of logic levels of sensor signals S1 to S3 is normal, whenever the logic levels of the sensor signals S1 to S3 are changed. Each of the sensor signals S1 to S3 indicates a rotation position of a rotor. When the combination is not normal, the number of the level changes in each of the sensor signals S1 to S3 is counted. When the count value of any one of the sensor signals reaches a threshold value Cth, supply of power to a motor is stopped.