Plural vector magnetic flux meters are disposed based on predetermined relationships in their positions, to obtain vector magnetic flux at plural points. The obtained vector magnetic flux includes magnetic flux components in plural directions. Curved surface interpolation is carried out based on the magnetic flux components in the corresponding direction to obtain magnetic flux components for arbitrary points. Magnetic flux components in plural directions for an arbitrary point are obtained by selecting the obtained magnetic flux components for the arbitrary point belonging to a region which overlaps with all interpolation regions for corresponding magnetic flux components of the magnetic flux. Vector magnetic flux corresponding to a condition that differences between positions of pickup coils of corresponding vector magnetic flux meters and the predetermined position of the vector magnetic flux meter are supposed to be 0, are obtained based on the obtained magnetic flux components in plural directions and the relative positions of the pickup coils.