A rotation angle detection device for detecting the position of a motor rotation shaft of a motor used in electric power steering systems, comprising: a first sensor and a second sensor configured to output a first sensor signal and a second sensor signal in accordance with a rotation of a motor rotation shaft of a motor, respectively; an angular position calculation unit configured to calculate angular position information that represents an angular position of the motor rotation shaft based on the first sensor signal; an rotation number detection unit configured to detect a rotation number of the motor rotation shaft based on the second sensor signal 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 (52, 53, 54) configured to supply power to the first sensor, the second sensor, the angular position calculation unit, the rotation number detection unit, and the rotation angle calculation unit when a power switch is on, and 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, characterized by a first quadrant information calculation unit configured to calculate first quadrant information representing which of quadrants the angular position of the motor rotation shaft belongs to based on the angular position information, the quadrants being formed by dividing a rotation range of the motor rotation shaft by a natural number N equal to or greater than 2; a second quadrant information calculation unit configured to calculate second quadrant information representing which of the quadrants the angular position of the motor rotation shaft belongs to based on the rotation number information representing the rotation number in the unit of 1/N rotation; and a correction unit configured to correct the rotation number information in accordance with an comparison result of the first quadrant information and the second quadrant information. a first sensor and a second sensor configured to output a first sensor signal and a second sensor signal in accordance with a rotation of a motor rotation shaft of a motor, respectively; an angular position calculation unit configured to calculate angular position information that represents an angular position of the motor rotation shaft based on the first sensor signal; an rotation number detection unit configured to detect a rotation number of the motor rotation shaft based on the second sensor signal 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 (52, 53, 54) configured to supply power to the first sensor, the second sensor, the angular position calculation unit, the rotation number detection unit, and the rotation angle calculation unit when a power switch is on, and 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, characterized by a first quadrant information calculation unit configured to calculate first quadrant information representing which of quadrants the angular position of the motor rotation shaft belongs to based on the angular position information, the quadrants being formed by dividing a rotation range of the motor rotation shaft by a natural number N equal to or greater than 2; a second quadrant information calculation unit configured to calculate second quadrant information representing which of the quadrants the angular position of the motor rotation shaft belongs to based on the rotation number information representing the rotation number in the unit of 1/N rotation; and a correction unit configured to correct the rotation number information in accordance with an comparison result of the first quadrant information and the second quadrant information.