Radiation-induced conductivity (RIC) of nitrogen-doped and undoped 6H-SiC has been studied under 17 MeV proton irradiation. The proton irradiation generates defects in the samples, and dark-conductivity (sigma (d)) decreases because of the carrier removal (CR) due to the defects. A decrease of the sigma (d) is visible in the undoped sample. Contrary an almost constant sigma (d) against the irradiation is obtained in the doped sample. The RIC increases in proportion to the proton flux in the doped sample, but a sublinear flux dependence is observed in the undoped sample. Both the differences of the sigma (d) decrease and of the flux dependence between the doped and the undoped samples are ascribed to a large difference of carrier concentration due to doping. Photoconductivity suppression (PCS) under simultaneous photon/proton irradiation of the undoped 6H-SiC is presented as a characteristic phenomenon under high flux proton irradiation.