To accurately measure a magnetic flux density even during rotating.

The apparatus for measuring a magnetic field comprises a three-axis magnetic sensor TMM having sensitivity axes in directions x, y and z, a two-axis acceleration sensor having a sensitivity axis in the direction z so as to be rotatable at the x-axis used as a rotary axis. Thus, a magnetic flux density of each direction is calculated by obtaining a low frequency component of a product of the output of the x-axis sensor MMX, the output of the x-axis sensor MMY and the output of the z-axis sensor MMZ and a low frequency component of a product of the output of the z-axis sensor MMX and the output of the z-axis sensor AMZ, and correcting the respective frequency components according to an oblique angle and an angle of a precession.