The ESR spectrum for polypyrrole broadens continuously with the rise of the partial pressure of oxygen without changing its line shape. Our experiments reveal that the line width variation is attributed to physically adsorbed oxygen within the polypyrrole film. With the exchange interaction as well as the dipole-dipole interaction between electron spins of polypyrrole and of molecular oxygen under consideration, the ESR line width of polypyrrole is suggested to be directly proportional to the number of adsorbed oxygen. In view of the linear relation, we attempt to estimate adsorption parameters from the oxygen-pressure dependence of the line width, using a model similar to Langmuir's adsorption mechanism. Such an attempt is further applied to the time variation of the line width when the sample under vacuum is open to the atmosphere. The approaches are presumed to give reasonable estimates of the adsorption parameters and expected to apply to other paramagnetic systems.