Fundamental free electron laser experiments with helical wiggler of 2 cm pitc have been done by using high current, low energy electron beam pulse power machine, Reiden III (0.6MeV/40kA/1kA/150ns) at Institute of Laser Engineering, Osaka University. Oscillation frequency variations in one electron shot were measured, and the frequencies can be explained by the theory of Raman regime free electron laser. In the calculation, we considered the beam velocity variation due to beam potential drop and wiggler field gradient in beam cross-section. Frequency spectra of two oscillation frequency modes shifting closely in each other, were obseved simultaneously by slimming the drift tube and decreasing the beam energy. In this case, the frequency spread effect by the beam cross-section appears widely and the measuring frequency spreads were distributed in the region predicted by the theory. Spatial growth rate was obtained as 1 dB/cm with saturation level of 250 kW and spectral peak at 58 GHz.
