To reduce a production cost and improve a yield by eliminating a mechanically movable part from a wavelength detection part.

This physical quantity measuring system detects reflection light from one or more optical fiber Bragg gratings FBG1-FBG10 formed in an optical fiber allowing incidence of measurement light to measure a physical quantity in each position of the optical fiber Bragg gratings FBG1-FBG10. The reflection light from the respective optical fiber Bragg gratings FBG1-FBG 10 is made incident on waveguide Bragg gratings WBG1-WBG10 each allowing a change of a reflection wavelength according to at least two control voltages, reflection light caused by the waveguide Bragg gratings WBG1-WBG10 is made incident on a photodiode, and a wavelength of each the reflection light from the optical fiber Bragg gratings FBG1-FBG10 is measured on the basis of a logarithm of a ratio of photoelectric currents according to the respective control voltages.