A light source unit irradiates a gas cell disposed in a measurement region with linearly polarized light in which the direction of travel is a z-axis direction and the vibration direction of an electric field is a y-axis direction. A polarimeter detects optical characteristics of light passing through the gas cell. A magnetic field generator applies an artificial magnetic field, capable of varying an x-axis component, a y-axis component, and a z-axis component, to the measurement region. A calculation control unit generates a plurality of artificial magnetic fields, calculates a magnetization value or a value corresponding to the magnetization value on the basis of the detection results of the polarimeter, and calculates an original magnetic field present in the measurement region, using an artificial magnetic field when the magnetization value or the value corresponding to the magnetization value satisfies a condition for external value.