Magnitudes of the absorption cross sections of a nitrogen molecule are measured by the transmission method near the ionization K-threshold. Based on these data, the partial absorption NK cross sections are obtained and the oscillator strength of the πg shape resonance is determined. It is found that the oscillator strength of the πg-resonance increases as the optical density of nitrogen decreases. This phenomenon is interpreted as a result of the instrumental distortion effect (the thickness effect). By extrapolating to zero pressure, the oscillator strength of the πg shape resonance is determined to be 0.29. © 2002 MAIK "Nauka/Interperiodica".