α-(BEDT-TTF) 2KHg(SeCN) 4 is the new organic conductor which has been synthesized by Sasaki recently. Although it has similar crystal structure with (BEDT-TTF) 2KHg(SCN) 4, it does not show antiferromagnetic like transition down to 0.5 K. Therefore the comparison of the physical properties of these two substances is interesting. The cyclotron resonance of K-Se salt has been measured in the millimeter and submillimeter wave regions with the pulsed magnetic field up to 15 T at 1.8 K, and we observed only one cyclotron resonance with the effective mass of 0.96 m e. It can be attributed to α-orbit, and this effective mass turned out to be smaller than 2.0 m e obtained by SdH oscillation. The circular polarization measurement has also been performed and this resonance turned out to be hole active which is consistent with the band calculation.