[en] A Martian summer cloud was observed on the flank of Olympus Mons located 20 deg N and 137 deg W during the 1982 apparition. This cloud, hereafter referred to as the Olympus cloud, showed a diurnal variation of the apparent brightness over more than one month. The Olympus cloud appeared near the local noon (12 MLT; Martian local time), and its apparent brightness increased until 16 MLT. Based on the assumptions that the Olympus cloud is composed of H₂O ice and that the Martian atmosphere contains a constant amount of dust equivalent to the optical thickness of 0.4, we estimated the optical thickness of the Olympus cloud with the aid of the discrete-ordinate method. We found that the progression of the optical thickness of the H₂O ice cloud against the MLT shows the peak value of about 0.5 near 14 MLT. This peak value corresponds to 0.7 precipitable micrometer. (author)