Microwave output power from a high-power helium-filled backward wave oscillator (BWO) operating at X band is measured as a function of the strength of guiding magnetic field B and helium gas pressure. Tenuous helium gas filling the slow wave structure is ionized by an injected electron beam with typical parameters of energy 100 keV and current 0.5 kA. The enhancement of output power from the present gas-filled BWO is less than those reported in some previous publications. Near B=0.8 T, we observe the minimum value of output that is judged to result from cyclotron absorption caused by resonant interaction of the fast electron cyclotron mode with the Cherenkov radiation. On both sides of the absorption, two peak outputs near B=0.5 and 1.0 T are measured. In the former case, the output is enhanced compared with vacuum case by the presence of helium gas with pressure of a few mtorr. In the latter case, on the other hand, the output monotonously decreases with the gas pressure. The output powers for the latter case are larger than those of the former case. Both cases are observed repeatedly for various experimental conditions. The physical reasons for the observed results are discussed.