A displacement detecting apparatus is provided with a self-position detector for detecting a position and an orientation of the apparatus relative to a preset reference position and a preset reference orientation. Displacement of a rail at an upper surface reference point and a side surface reference point relative to a preset reference position is calculated from imaged image data of the surfaces of the rail using an optical cutting method, and the calculated displacement of the rail is corrected using data obtained from the self-position detector, thereby compensating for errors caused by differences in position and orientation of the apparatus associated with a vehicle running motion. By doing so, the displacement detecting apparatus is superior in maintenance and unlikely to cause measurement errors.