The atmospheric neutrino background for proton decay via p→e+π0 in ring imaging water Cherenkov detectors is studied with an artificial accelerator neutrino beam for the first time. In total, 3.14×105 neutrino events corresponding to about 10 megaton-years of atmospheric neutrino interactions were collected by a 1000 ton water Cherenkov detector (KT). The KT charged-current single π0 production data are well reproduced by simulation programs of neutrino and secondary hadronic interactions used in the Super-Kamiokande (SK) proton decay search. The obtained p→e+π0 background rate by the KT data for SK from the atmospheric neutrinos whose energies are below 3 GeV is 1.63+0.42−0.33(stat)+0.45−0.51(syst)(megaton-year)−1. This result is also relevant to possible future, megaton-scale water Cherenkov detectors.