separated from the first region a 1 the second region a 2 is a region where an optical modulation operation is not performed accordingly the light l 22 incident to the optical modulator 40 a from the object 60 is reflected to the optical coupler 24 without any optical modulation the optical modulator 40 a may be for example a digital micro mirror device dmd or a spatial light modulator slm the dmd includes a plurality of mirrors and each of the micro mirrors may perform as a pixel since the optical modulator 40 a may perform as a grating a large amount of diffracted light may be generated from the optical modulator 40 a a specific diffracted light of the diffracted lights for example a fourth diffracted light is used for obtaining a tomographic image of the object 60 accordingly diffracted lights that are not used for obtaining a tomographic image of the object 60 may be diffracted light unnecessarily generated and to remove the diffracted light unnecessarily generated the second optical system 40 includes the first grating 40 b the first grating 40 b may have a groove density a slit density that is the same as that of the optical modulator 40 a therefore a specific diffracted light generated from the optical modulator 40 a enters the object 60 and the diffracted light unnecessarily generated may be removed.