  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 $\pi^{+}$ production in the resonance
region ($W<2$ GeV/$c^2$) relative to the charged-current quasi-elastic 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.
