Electron impact emission cross sections of Ar were measured for the 4p → 4s, 5p → 4s and 4s → 3p transitions from the threshold to 1000 eV by a polarization-free method. Two different methods were used in determining absolute emission cross sections for the first resonance lines. The first method is to measure the cascade cross sections to the 4s state by detecting visible to near-infrared radiation and the total cascade cross section was then combined with the level excitation cross section to the 4s state to give the emission cross section for the resonance line. The second method is to measure the vacuum ultraviolet sensitivity of the apparatus and an absolute emission cross section for the resonance line was obtained by comparing its spectral intensity with that of the Lyman-α radiation produced from H2 by electron impact. The two results showed good agreement within the experimental errors. The level excitation cross sections to the 1s2 and 1s4 states were obtained by subtracting the total cascade cross section to these levels from the corresponding emission cross sections.