PURPOSE: To obtain a highly accurate interference graphic with vibration or the like applied by integrating a first part branched into a sample optical path and a reference optical path, a first return mirror and a second return mirror to return in the reference optical path and a second part mirror in rigidity.

CONSTITUTION: A first prism 3 is arranged in the direction of a parallel lighting luminous flux travels and a branch half mirror 4 and a return mirror 5 are formed as integrated single optical part so that an outgoing sample optical axis 7 and a reference optical axis 6 parallel the optical path of an incident light. A sample 8 is arranged on the optical axis 7 while a second prism 9 is arranged along the direction of incidence of two luminous fluxes passing through the optical axes 6 and 7. Like the prism 3, in the prism 9, a coincidence half mirror 10 and a return mirror 11 are integrated to make a single optical part and the prism 3 is arranged being rotated by 180° so that sample light coincides with the reference light on the half mirror 10. An interference graphic obtained on the half mirror 10 is observed at an observing position 13 through a lens 12.