An electric power steering apparatus has a rotation detection device that includes a sensor section that detects a rotation of a motor and outputs a mechanical angle and a count value, and a signal obtainer that obtains the mechanical angle and the count value from the sensor section. The rotation detection device also includes an absolute angle calculator that calculates an absolute angle based on the mechanical angle and the count value and a storage area for storing a reference value that is used for correcting calculation errors in the absolute angle.