A differential refractometer in which: light from a light source is condensed on a lens; the light thus condensed is guided to a cell which houses a sample of which refractive index is to be measured and a reference of which refractive index is used as a reference value, the sample and the reference being housed as separated from each other in the cell; the light having passed through the cell is guided to an image sensor; and the amount of light deflection due to the difference in refractive index between the sample and the reference is measured, thereby to obtain the refractive index of the sample. The differential refractometer comprises: a light permeable member disposed at such a position that the light permeable member and the image sensor are optically conjugate with respect to the lens, the light permeable member carrying an image having at least two identification portions; and a spatial filter disposed between the light permeable member and the image sensor and adapted to spatially filter light portions which have passed through the two identification portions, the spatial filter causing at least one of the light portions thus spatially filtered to pass through the cell. The refractive index of the sample may be obtained based on the distance between the positions of images, on the detecting surface of the image sensor, of the two identification portions of the light permeable member. Such an arrangement may improve the measuring precision and reduce the number of component elements.