A physical quantity measuring apparatus utilizing optical frequency domain reflectometry, comprising: a tunable laser that emits measuring light; a first polarization-maintaining fiber with one end thereof connected with the tunable laser; a polarization-maintaining coupler connected with another end of the first polarization-maintaining fiber; a second polarization-maintaining fiber with one end thereof connected with the polarization-maintaining coupler, and another end thereof being a referential reflecting end; a third polarization-maintaining fiber with one end thereof connected with the polarization-maintaining coupler; a sensor consists of fiber Bragg gratings formed at a core of the third polarization-maintaining fiber; a fourth polarization-maintaining fiber with one end thereof connected with the polarization-maintaining coupler; a photodiode connected with the polarization-maintaining coupler via the fourth polarization-maintaining fiber, and detects Bragg reflected light from the sensor and reference light from the referential reflecting end; a controller that detects a modulation of an interference intensity between the Bragg reflected light and the reference light, based on an intensity change of multiplexed light of the Bragg reflected light and the reference light detected by the photodiode; an incidence part that inputs the measuring light to both two orthogonal polarization axes of the second polarization-maintaining fiber and two orthogonal polarization axes of the third polarization-maintaining fiber; and an optical path-length adjuster that is arranged on the third polarization-maintaining fiber, and keeps the optical path-length of the Bragg reflected light from the two orthogonal polarization axes at the sensor at a constant length, wherein the incidence part being provided on the first polarization-maintaining fiber, or on both the second polarization-maintaining fiber and the third polarization-maintaining fiber. a tunable laser that emits measuring light; a first polarization-maintaining fiber with one end thereof connected with the tunable laser; a polarization-maintaining coupler connected with another end of the first polarization-maintaining fiber; a second polarization-maintaining fiber with one end thereof connected with the polarization-maintaining coupler, and another end thereof being a referential reflecting end; a third polarization-maintaining fiber with one end thereof connected with the polarization-maintaining coupler; a sensor consists of fiber Bragg gratings formed at a core of the third polarization-maintaining fiber; a fourth polarization-maintaining fiber with one end thereof connected with the polarization-maintaining coupler; a photodiode connected with the polarization-maintaining coupler via the fourth polarization-maintaining fiber, and detects Bragg reflected light from the sensor and reference light from the referential reflecting end; a controller that detects a modulation of an interference intensity between the Bragg reflected light and the reference light, based on an intensity change of multiplexed light of the Bragg reflected light and the reference light detected by the photodiode; an incidence part that inputs the measuring light to both two orthogonal polarization axes of the second polarization-maintaining fiber and two orthogonal polarization axes of the third polarization-maintaining fiber; and an optical path-length adjuster that is arranged on the third polarization-maintaining fiber, and keeps the optical path-length of the Bragg reflected light from the two orthogonal polarization axes at the sensor at a constant length, wherein the incidence part being provided on the first polarization-maintaining fiber, or on both the second polarization-maintaining fiber and the third polarization-maintaining fiber.