Provided is a differential refractive index detector having a light receiving element, a zero glass, a zero glass driving unit and a storing portion, and is capable of performing purging operation thoroughly based on a unified standard. The light receiving element receives a measuring light passing through cells (S, R) to generate a slit image. The zero glass makes the slit image parallelly move on the light receiving element. The zero glass driving unit makes the zero glass rotate. The storing portion stores a rotating angle of the zero glass when the same solution fills up the two cells (S, R). When a purging operation for replacing a reference solution in the flow cell is performed, the stored rotating angle is taken as a standard value for being compared with a current rotating angle of the zero glass. If the two angles are the same, the purging operation is finished.