By the end face of the refractive index dispersion lens 16, to be inclined a predetermined angle to the incident surface 17a of the prism 17, and filled with an adhesive B therebetween, and is fitted with a prism 17 to the refractive index dispersion lens 16. In this way, the amount of light reflected from the bottom surface 17c of the prism 17 is reduced according to the inclination angle of the incident surface 17a, and is formed with the light reflected from the bottom surface 17c of the prism 17, and the reflected light of the reference beam Since the interference signal is weakened, the image signal is reduced MK, distinction between the image signal WS based on the reflected light from the measurement target is so attached easily.屈折率分散レンズ16の端面に対して、プリズム17の入射面17aを所定角度傾けて、その間に接着剤Bを充填することで、屈折率分散レンズ16にプリズム17を取り付けている。このようにすれば、入射面17aの傾き角に応じてプリズム17の底面17cからの反射光量が減少するので、プリズム17の底面17cからの反射光と、参照光の反射光とで形成される干渉信号が弱まり、画像信号MKが小さくなるため、測定対象からの反射光に基づく画像信号WSとの区別が容易に付くようになる。
