To provide a measurement apparatus for stabilizing the sensitivity of a probe, and surely measuring an electric field strength or a magnetic field strength.

The measurement apparatus includes a laser light source 12, a polarization controller 13 for converting a polarization state of a light, an optical fiber 11 for guiding the light emitted from the polarization controller 13, the probe 19 provided in the optical fiber 11, a photodetector 111 for entering the light emitted from the probe 19 through the optical fiber 11, and a polarization monitor 112 for detecting the polarization state of the light entering into the photodetector 111. The probe 19 has a polarizer 15, an EO crystal 17 for changing a refractive index, and a reflection mirror 18 for reflecting a light transmitted through the EO crystal 17. The polarization controller 13 converts the polarization state of the light entering from the laser light source 12 to cause the polarization state of the light entering into the optical fiber 11 to be a polarization state for canceling a change in the polarization state which is applied to the light within the optical fiber 11.