Single charged pion production in charged-current muon neutrino interactions with carbon is studied using data collected in the K2K long-baseline neutrino experiment. The mean energy of the incident muon neutrinos is 1.3 GeV. The data used in this analysis are mainly from a fully active scintillator detector, SciBar. The cross section for single π+ production in the resonance region (W<2  GeV/c2) relative to the charged-current quasielastic cross section is found to be 0.734+0.140−0.153. The energy-dependent cross section ratio is also measured. The results are consistent with a previous experiment and the prediction of our model.