A rotation angle detection device including: an angular position calculation unit configured to calculate angular position information representing an angular position of the motor rotation shaft based on a first sensor signal of the first sensor; an rotation number detection unit configured to detect a rotation number of the motor rotation shaft based on a second sensor signal of a second sensor and outputs rotation number information representing the rotation number; a rotation angle calculation unit configured to calculate rotation angle information indicating a rotation angle of the motor rotation shaft based on the angular position information and the rotation number information; and a power supply unit configured to stop supplying power to the first sensor, the angular position calculation unit and the rotation angle calculation unit and to supply power to the second sensor and the rotation number detection unit when the power switch is off.