Niobium thin films with extremly low resistivity (14.7 μΩ cm) have been deposited by tetrode sputtering in Ar gas purified by the getter action of titanium sublimation pump. The effect of gaseous impurities on the resistivity, superconducting transition temperature, and crystal structure were investigated. The superconducting transition temperature of such films was enhanced to 9.8±0.1 K and the temperature coefficient of resistance was increased to +3.0×103 ppm/deg.