(A prepring publication) Establishment of mechanism of electron pairing with a lower limit of binding energy is necessary for understating of superconductivity. Due to conservation of wavevector, photon absorption/emission by an electron in crystal can only be allowed across at least on band gap, which is also true for virtual photon absorption/emission inducing electron pairing in crystal. Therefore, it is clearly explained that electron pairs, formed by virtual stipulated transition, can only exist between electrons across a band gap, with a binding energy no smaller than the width of the band gap.