The magnetic field strength is calculated numerically for three types of coils, i.e., a cylindrical, a cubic and a wave-like coils. It is possible from the calculated results to test whether each coil meets two requirements of the coil winding which is free from the magnetic force and the configuration of coil winding for providing the high β value or not. The cylindrical coil fulfills the requirement of coil winding which is free from the magnetic force, while the cubic coil fulfills the requirement of providing high β. It should be stressed that the port distorts markedly the initially intended magnetic field.