In optical atmospheric (OA) pulse position modulation (PPM) code division multiple access (CDMA) systems, the conventional scheme which applies the maximum-value detection has a serious problem that the multiple access Interferences (MAIs) become large due to scintillations. In this paper, to mitigate both effects of scintillations and MAIs, we propose a symbol decision scheme by using the comparing detection. In our proposed scheme, the minimum output In all weighted positions Is detected In each PPM slot. Then, the maximum-value out of the minimum outputs Is detected, and, the symbol corresponding to the PPM slot In which the output has the maximum-value Is decided. Since the received symbol Is correctly decided as long as there Is at least one weighted position without MAIs In other PPM slots, the proposed scheme can mitigate both effects of MAIs and scintillations. We show that the proposed scheme can Improve the bit error rate performance.