[en] We study single and double β decay for N approx. = Z nuclei, using an extended P + QQ interaction with four force strengths. The Gamov-Teller (GT) transition is examined using the proton-neutron QRPA (pn-QRPA) in many j shells and using a single j shell model calculation. First, we calculate the single β-decay strength. The integrated strengths of N approx. = Z nuclei in the fp shell region are reproduced well by the pn-QRPA, although some quenching factor is necessary for several cases. The model accounts for the experimental observation that the GT strength increases with decreasing |N - Z|. The single j shell model calculation indicates that the GT strength becomes very large at N = Z. Second, we perform calculations for two-neutrino double β-decay transition strengths for ⁷⁶Ge and ⁸²Se. The results suggest fairly good applicability of the extended P + QQ interaction to single and double β decay. (author)