An optical system of a shape measuring instrument includes a laser diode, a first optical system irradiating an object to be measured with laser beam, a second optical system focusing reflected light from the object to be measured, and a CCD line sensor portion for detecting a laser beam from the second optical system, where the first optical system includes an optical path displacing unit for displacing an optical path for laser beam, the optical path displacing unit includes a glass plate rotating about a rotating axis extending in a direction perpendicular to a displacement plane of the optical path, a rotating unit for the glass plate, and a rotating unit controller, and the rotating unit controller causes a rotation angle of the rotating unit to coincide with a rotation angle of the rotating unit obtained when the maximum value of an amount of light received by the CCD line sensor according to rotation of the rotating unit is measured by the rotating unit controller.