To easily measure, for example, the internal diameter, inside shape, and surface roughness of a measured body which are hard to measure without contacting.

The instrument is equipped with a light projection part 1 which irradiates the inside of a measured body with measurement light, an irradiating light deflection part 2 which deflects measurement light irradiating the inside of the measured body to irradiate a measured place inside the measured body, a reflected light deflection part 3 which deflects the measurement light reflected at the measured place to make the light incident on a photodetecting lens 4, and a position detecting element 5 which detects the imaging position of the measurement light converged by the photodetecting lens through photoelectric conversion.