In general, perovskite crystals are composed of monovalent cations, divalent metal cations, and halogen anions. Most reported high-efficiency perovskite solar cells contain toxic lead. To solve this problem, there is a need to substitute lead with low-toxic elements. In this study, we focus on copper (Cu) and investigate the effect of Cu substitution on the electronic structures and photovoltaic properties. From the band structures obtained by first-principles calculations, the energy levels of the Cu-d orbital, which formed slightly above the valence band maximum, are predicted to work as an acceptor or a defect level. Devices in which 2 % Cu compound was added to the perovskite precursor solution showed higher device performance than standard devices. On the other hand, the short-circuit current density decreased with increasing the Cu composition. The calculations and experiments indicated that the energy level of the Cu-d orbital would work as a defect level and that carrier recombination would decrease the current density. Combining first-principles calculations and experiments, the effect of Cu substitution in methylammonium-based perovskite solar cells was clarified.