Provided is a physical quantity measuring device of optical frequency range reflection measuring type, which comprises a first polarization holding fiber connected at one end with a tunable laser and at the other end with a polarization holding coupler, a second polarization holding fiber connected at one end with the polarization holding coupler and having a reference reflecting terminal at the other end, a third polarization holding fiber having a sensor formed of a fiber Bragg grating at a core and connected at one end with the polarization holding coupler, a fourth polarization holding fiber connected at one end with the polarization holding coupler, an incident unit for making a measuring light incident on both the two orthogonal polarization axes of the second polarization holding fiber and the two orthogonal polarization axes of the third polarization holding fiber, and an optical path length adjusting unit arranged in the third polarization holding fiber for making constant the optical path lengths of the Bragg reflection lights from the two orthogonal polarization axes in the sensor. The incident unit is arranged either at the first polarization holding fiber or at both the second polarization holding fiber and the third polarization holding fiber.