We aim to understand foreign gas effect on A-band of oxygen that is optical forbidden b1Σg+(v=0)←X3Σg-(v=0) transition. To measure the absorption of weak A-band, a high pressure absorption spectroscopy system was constructed. We found absorbance around A-band of O2/M (N2, CO2, SF6, Ne, Ar, Kr, Xe) gas mixture were larger than that of pure O2. This enhancement by foreign gases was discussed from a view point of heavy atom and polarizability effects under the light of the presented mechanism of the heavy atom effect. We performed quantitative analysis on the enhancement by noble gases, which are monoatomic molecules, as a foreign gas. The polarizability effect was examined by using SF6 as a foreign gas, which is a simple molecule with large polarizability.