Poloidal magnetic field measured outside a plasma column consists of externally applied vertical field and the field produced by a plasma current. if we assume that the plasma current is replaced by a finite number of loop-currents and that the externally applied vertical field is produced by externally located loop-currents, the externally applied vertical field can be separated from the poloidal magnetic field due to the plasma current with the help of a method of least squares. Consequently, the value of the plasma current, flux surfaces of the externally applied vertical field and the plasma position can be obtained.