To update offset of a magnetic data measuring device.

A device 1 comprises a component which generates an internal magnetic field, a three-dimensional magnetic sensor 60 and a CPU 10. On the basis of a plurality of magnetic data items successively outputted from the three-dimensional magnetic sensor 60, the CPU 10 updates offset that is coordinates of three axes representing components of the magnetic field generated by the component, from former offset to new offset. The new offset is calculated as coordinates of the center of a spherical surface minimizing a magnitude of an error vector with, as elements, a first error between a spherical surface having coordinates indicated by a plurality of magnetic data items in the vicinity of the surface and coordinates indicated by magnetic data, a second error obtained by multiplying, by a first coefficient, an inner product of a normalized eigen-vector corresponding to a minimum eigen-value of a covariance matrix representing the variance of the plurality of magnetic data items and an update vector indicating the coordinates of the center of the spherical surface watched from the former offset, and a third error obtained by multiplying, by a second coefficient, an inner product of a normalized eigen-vector corresponding to an intermediate eigen-value of the covariance matrix and the update vector.