In a magnetic field measurement apparatus, a light source irradiates a gas cell with linearly polarized light serving as pump light and probe light in a Z axis direction, and a magnetic field generator applies alternating magnetic fields which have the same cycle and different phases to the gas cell in each of X axis and Y axis directions. A calculation controller calculates a magnetic field C (Cx, Cy, Cz) of a measurement region using X axis and Y axis components Ax and Ay of the alternating magnetic fields, and a spin polarization degree Mx corresponding to a measurement value W− from a magnetic sensor.