In a magnetic data processing device, a magnetic data input part sequentially receives magnetic data output from a three-dimensional (3D) magnetic sensor. A storage part stores a plurality of the magnetic data as a data set of statistical population. An acceleration data input part receives acceleration data output from a 3D acceleration sensor. A reliability determination part derives a reliability index that is a function of an angular difference between a direction of a line perpendicular to an approximate plane representing a distribution of the data set of the statistical population and a direction of acceleration represented by the acceleration data.