To provide a laser length measuring machine capable of precisely measuring displacement of a curved surface such as a cylindrical side surface by minimizing the influence of fine irregularities of a measuring surface and eliminating the influence of a relatively large flaw locally existing in the measuring surface.

This length measuring machine comprises a beam dividing means 100, an interferometer 2 dividing a divided laser beam to a first beam and a second beam and composing the first beam reflected by a reference mirror 3 and returned therefrom with the second beam reflected by a measuring object 5 and returned therefrom; an objective lens optical system 4; a condenser lens 110 converging interference light emitted from the interferometer 2; a light detector 6, and a length measuring circuit 7. Further, a beam interruption means 120 interrupting an optional part of the divided first beam or second beam is provided between the beam dividing means 100 and the interferometer 2.