Measurement system and measuring method

A ranging apparatus transmits an optical input signal to an optical signal input/output unit mounted on a measurement target. The optical signal input/output unit receives the optical signal and transmits an optical output signal applied with an optical change, to the ranging apparatus. The ranging apparatus receives the optical output signal, measures a propagation distance from a light source a the light receiving unit through the optical signal input/output unit, and measures a relative position of the optical signal input/output unit based on the propagation distance. Thus, a distributed aperture radar is realized from the ranging apparatus and the optical signal input/output unit.