Ann-CdS/p-CdTe solar cell module constructed by complete seal of solar cell formed on a glass substrate by screen-printing and sintering process, was subjected to a heavy sunshine weather-meter test. The conversion efficiency of the module began to degrade after 50 days. Chemical analyses on gases in the sealed module showed that decrease in oxygen down to 2 vol.% had caused the degradation. Inversely, when sufficient oxygen was supplied to the solar cell by breaking the seal, the efficiency recovered up to its initial value within several days even at room temperature. Estimation of reaction velocity at room temperature suggested that the present recovery phenomenon was the same as increase inp-type characteristics by heat treatment in air in other fabrication process of CdS/CdTe junction. Easiness and reversibility of transformation from oxygen deficient to sufficient states in CdTe, strongly suggest that oxygen in CdTe induces a single acceptor which was confirmed in oxygen doped ZnSe.