Geomagnetic field measurement device, offset determination method, and computer readable recording medium therefor

A geomagnetic field measurement device has a three-dimensional magnetic sensor and a storage device that stores plural pieces of magnetic data sequentially output from the three-dimensional magnetic sensor. The geomagnetic field measurement device calculates, based on the pieces of magnetic data, a distortion estimation value indicating a degree of difference in shape between a three-dimensional figure defined so that the plural sets of coordinates indicated respectively by the plural pieces of magnetic data are distributed adjacent to a surface thereof and a spherical surface, and determines whether there is distortion in the shape of the three-dimensional figure based on the distortion estimation value, to update an offset for correcting each of the plural pieces of the magnetic data in a case in which it is determined that there is no distortion.