How ordinarily existing scatters determine electric resistivity in a superconducting state is calculated here in order to explain why the electric resistance vanishes suddenly at the critical temperature Tc of a superconductor. Three mechanisms are considered: (1) non-magnetic impurity scattering, (2) phonon scattering, and (3) magnetic (spin-flip) scattering. If one tentatively calculates conductivity as the response to an applied electric field and tries to solve a finite current response under a vanishing electric field inside a superconductor, then the solved conductivity should be infinite. It is possible to show that the calculated conductivity is infinite for the former two scattering mechanisms and finite for the last. The calculations are based on the Kubo formula and are carried out by using retarded Green's functions taking into account any scattering to the lowest order and a random phase approximation. The basis of this theory are the states defined by the Fermi-gas model with an attractive electron correlation as well as electron-scatterer interaction.
