A rotation angle detecting apparatus comprises a bearing holder, a rotation shaft rotatably supported by the bearing holder, a shaft portion space formed in the rotation shaft, a bearing holder space formed in the bearing holder, an angle detection pattern in the shaft portion space, a reference pattern and an image sensor in the bearing holder space, an optical system which exists across the shaft portion space and the bearing holder space and forms a projection image of the angle detection pattern and a projection image of the reference pattern on the image sensor, a photodetection switching means for selectively projecting the projection images of the angle detection pattern and the reference pattern onto the image sensor, and an arithmetic device for calculating a rotation angle of the rotation shaft based on a deviation between the reference pattern and the angle detection pattern received by the image sensor.