The invention is directed to the provision of a device and method for measurement of angle of rotation for measuring in a non-contacting fashion the concentration of an optically active substance such as a sugar, amino acid, vitamin, or the like dissolved in a sample. According to the invention, A coherent beam is split into two beams, one being an object beam and the other being a reference beam, the object beam passed through the sample is converted by a quarter wave plate into mutually perpendicular polarization components having therebetween a phase difference proportional to the angle of rotation produced by the sample, and the angle of rotation produced by the sample is obtained from the phase difference occurring between beat signals produced by causing the object beam with the reference beam. Alternatively, a coherent beam is directed into the sample, the beam passed through the sample is converted by a quarter wave plate into mutually perpendicular polarization components having therebetween a phase difference proportional to the angle of rotation produced by the sample, and the angle of rotation produced by the sample is measured based on the phase of a beat signal produced by causing the mutually perpendicular polarization components to interfere with each other by means of a polarizer.