A slow-wave electron cyclotron maser consisting of an oversized corrugated waveguide and an axially injected electron beam has been demonstrated experimentally. At resonance, the output rf power is strongly enhanced. Maximum output power of 200 kW with an electronic efficiency of 4% at 19.5 GHz is obtained for an electron beam with voltage of 35 kV and current of 150 A. The oscillation is observed to be a periodic TM(01) mode. The enhancement of microwave radiation is estimated to be caused by the combined resonance of Cherenkov interactions and an anomalous Doppler shifted electron cyclotron interaction.