Upstream veto counter for the K2K long baseline neutrino oscillation experiment

We have constructed and tested the upstream veto counter for the K2K long baseline neutrino oscillation experiment. The upstream veto counter was constructed in such a way that two VENUS scintillators were read out via a single photomultiplier tube at each end. Timing resolutions and efficiencies of the veto counter were better than 1.0 ns and 99% in the active region of the scintillators, respectively. Energy resolutions of the veto counter are also better than 20% between the far end and the center of the scintillators. We conclude that the upstream veto counter meets requirements to reject backgrounds such as cosmic muons and beam-related muons from the upstream water Cherenkov counter and to separate beam-induced electrons from muons.