The mechanism of magnetoconductance (MC) in organic solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) remains unclear. Recombination of polaron pairs in the P3HT:PCBM organic solar cell is important in view of MC as well as charge transportation. To examine whether the polaron-pair model explains the mechanism of MC in the P3HT:PCBM system, we used an equivalent circuit model in which the internal shunt resistance represents the polaron-pair recombination. The almost identical dependence of the MC and the magnetic field effect on the internal shunt resistance for the P3HT:PCBM solar cells with various polaron-pair recombination rates clearly shows that the MC is ascribed to the polaron-pair recombination. Furthermore, the fact that the MC is virtually unaffected by light intensity below 100 mW/cm(2) confirms that the polaron pairs responsible for the MC are geminate. The circuit model provides a useful instrument for probing the polaron-pair recombination in the system where excitons are effectively quenched.