PURPOSE: To obtain a magnetic field measuring device having high accuracy, which is not influenced by an optical transmission loss and drift of a light-electricity transducer, by measuring a Faraday rotation angle F by use of optical superheterodyne detection.

CONSTITUTION: A two frequency laser 11 emits a beam having an electric field Ex of optical frequency ω linearly polarized in the (x) axis direction, and an electric field Ey of optical frequency ω+Δω linearly polarized in the (y) axis direction. The beam is divided into two by a beam splitter 12, and one is made incident to λ/4 wavelength 15, and the other beam is directed to an analyzer 13. The beam emitted from the λ/4 plate 15 passes through a Faraday rotator 16. An analyzer 17 makes only an electric field Ex" in the (x) axis direction pass through, and a photodiode 18 converts it to an electric signal Ix being proportional to optical intensity Ex"2. The analyzer 13 makes linear polarized light whose azimuth is 45° from the (x) axis pass through, and a photodiode 14 converts it to an electric signal Iref being porportional to optical intensity Eref2. By processing Ix and Iref, a detected value being completely proportional to a Faraday rotation angle F can be obtained.