A refractometer for measuring refractive index of a sample comprising a prism having an interface contacting the sample, a light source that radiates light towards the interface from an entrance face of the prism, and a photoelectric sensor for receiving light reflected at the interface and directed outward from an exit face of the prism. The light source and the photoelectric sensor are secured to the prism. The refractometer comprises a sample stage arranged surrounding the interface surface. This sample stage provides a non-adhesive coating formed on the surface thereof. The refractometer comprises filter means arranged between the interface surface and the photoelectric sensor. This filter means includes a wavelength filter that selectively allows transmission of light having a wavelength within a prescribed region, including a wavelength of light of the light source.