A small-size optical rotation measuring device for detecting an organism, a tissue, blood, or molecules having rotatory power and determining the content thereof with high accuracy and an optical rotation measuring method for detecting an organism, a tissue, blood, or molecules having rotatory power and determining the content thereof with high accuracy. A nonreciprocal optical system is disposed in a loop optical path of a ring optical interferometer, and thereby light beams of circularly polarized modes orthogonal to each other are propagated in opposite directions through a sample to be measured. The wavelength of the light beams from a light source is in a wavelength region where the loss by the nonreciprocal optical element is low. A signal processing technique for phase-modulation optical fiber gyro having the highest resolution among ring interferometers is applied.