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, to the gas cell, a magnetic field Ax which is a time function f(t) having the amplitude A0 taking n fixed values fi (where i=1, . . . , and n), and a magnetic field Ay which is a time function g(t) having the amplitude A0 taking m fixed values gj (where j=1, . . . , and m) 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 the X axis and Y axis components Ax and Ay of an artificial magnetic field A, and a spin polarization degree Mx corresponding to a measurement value W− from a magnetic sensor.