To provide a wave front measuring interferometer unit provided with a compact optical system of simple constitution, and capable of regulating easily the optical system, and a light beam measuring instrument and method capable of measuring a wave front of a light beam and measuring a characteristic of a light beam spot.

This light beam measuring instrument 10A is provided with a beam splitter 13 for splitting the light beam emitted from a light source part 11 into two beams, a semi-transparent reflection face 15a for reflecting one portion of the separated one light beam as a inspected light beam to a direction reverse to an incident direction, and a reflection type reference light generating means 23 for converting one portion of the transmitted light beam transmitted through the semi-transparent reflection face 15a into a wave front-faired reference light beam to be output. The wave front of the light beam, and the characteristic of the light beam spot are measured by such a manner.