PURPOSE: To measure a refractive index in a real time with high accuracy, by a method wherein laser beam is split into two beams and one of them is allowed to propagate through a specimen while the other beam through a reference medium and both beams are mixed to be guided to a detection part.

CONSTITUTION: Laser beam from a light source 15 is split into two beams by a beam splitter (BS)16 and both of them receive frequency modulation with frequencies α1, α2 by beam modulators 17, 18 and converted to polarized waves crossing at right angles by polarizers 19, 20 while these waves are incident to an optical fiber 25 through mirrors 21, 22, a beam mixer 23 and a convex lens 24. The beam emitted from the optical fiber 25 is split into polarized waves with frequencies α2, α1 crossing at right angles by a microlens 26 and a split type beam detector 27 and incident to a mirror 32 through a specimen holding part (reference medium) 30 and a specimen 31. Reflected beams advance reversely and mixed by BS33 to be incident to a detector 35 through a beam dedector 34 having an angle of 45° to the polarizing direction of the mixed wave. The phase difference of beams with frequencies α1, α2 is detected by the detector 55 and the difference of the refractive index of the specimen to that of the reference substance 30 is measured in a real time with high accuracy.