An oblique incidence interferometer for applying a light at a certain angle from the normal to a measurement surface of a target to be measured and measuring a light reflected from said target, comprising: a light source; a beam splitter/synthesizer unit (14,16, 17, 17', 50,55-57, 60) configured to split the light from said light source into a measurement light to be applied to said target and a reference light serving as the measurement reference, and configured to orthogonalize the polarization directions of said measurement light reflected from said target and said reference light and synthesize said lights to yield a synthesized light, a beam splitter unit configured to split said synthesized light into a plurality of split lights; a plurality of imaging units (35A - 35C) configured to capture a plurality of interference fringe images formed in accordance with said plurality of split lights; a 1/4-waveplate provided on either one of an entry side and an exit side of said beam splitter unit; a plurality of polarizers (34A - 34C) provided on imaging surfaces of said plurality of imaging units (35A-35C) and arranged having respective different polarization axes; and an operation unit configured to compute a surface form of said target based on said interference fringe images captured by said imaging units (35A - 35C) and resulted from said plurality of split lights having phases shifted through said polarizers (34A - 34C), characterized in that said beam splitter/synthesizer unit (50, 17, 55-57, 60) includes means (50, 60) for splitting the light from said light source into two light beams (L 1, L 2) having orthogonal polarization planes and generating two collimated beams (L 1 ', L 2 ') having different optical axes and a certain diameter, a triangular prism operative to transmit one of said two collimated beams therethrough and reflect the other therefrom, thereby splitting said two collimated beams into a measurement light (L 2 'B) and a reference light (L 1 'A) serving as the measurement reference, and operative to synthesize said measurement light with said reference light to yield a synthesized light, and a spatial filter unit operative to pass said synthesized light therethrough and reject a noise light (L 1 'B) caused from one of said two collimated beams reflected from said triangular prism and a noise light (L 2 'A) caused from the other of said two collimated beams transmitted through said triangular prism. a light source; a beam splitter/synthesizer unit (14,16, 17, 17', 50,55-57, 60) configured to split the light from said light source into a measurement light to be applied to said target and a reference light serving as the measurement reference, and configured to orthogonalize the polarization directions of said measurement light reflected from said target and said reference light and synthesize said lights to yield a synthesized light, a beam splitter unit configured to split said synthesized light into a plurality of split lights; a plurality of imaging units (35A - 35C) configured to capture a plurality of interference fringe images formed in accordance with said plurality of split lights; a 1/4-waveplate provided on either one of an entry side and an exit side of said beam splitter unit; a plurality of polarizers (34A - 34C) provided on imaging surfaces of said plurality of imaging units (35A-35C) and arranged having respective different polarization axes; and an operation unit configured to compute a surface form of said target based on said interference fringe images captured by said imaging units (35A - 35C) and resulted from said plurality of split lights having phases shifted through said polarizers (34A - 34C), characterized in that said beam splitter/synthesizer unit (50, 17, 55-57, 60) includes means (50, 60) for splitting the light from said light source into two light beams (L 1, L 2) having orthogonal polarization planes and generating two collimated beams (L 1 ', L 2 ') having different optical axes and a certain diameter, a triangular prism operative to transmit one of said two collimated beams therethrough and reflect the other therefrom, thereby splitting said two collimated beams into a measurement light (L 2 'B) and a reference light (L 1 'A) serving as the measurement reference, and operative to synthesize said measurement light with said reference light to yield a synthesized light, and a spatial filter unit operative to pass said synthesized light therethrough and reject a noise light (L 1 'B) caused from one of said two collimated beams reflected from said triangular prism and a noise light (L 2 'A) caused from the other of said two collimated beams transmitted through said triangular prism.