A rotation angle detecting method includes measuring a current flowing through a motor, and obtaining a rotation angle of the motor based on a value of the measured current. The obtaining of the rotation angle of the motor includes obtaining the rotation angle in an inertial rotation of the motor based on a rotational angular velocity of the motor immediately before a first time point, a time period from the first time point to a second time point, and a time period from the second time point to a third time point. The first time point is a time point at which driving of the motor is stopped, the second time point is a time point at which a polarity of the value of the measured current is inverted, and the third time point is a time point at which the value of the measured current becomes zero.