Rhenium and Osmium abundances were determined on molybdenites from eighteen ore deposits in Japan; fifteen vein-type, a skarn-type, a greisen-type, and a pegmatite-type deposit. ReOs ages obtained for molybdenites from northeastern, central, and southwestern Japan are 100–130 Ma, 50–75 Ma, and ca. 10 Ma, respectively, reflecting the ages of regional igneous activities in individual areas.For vein-type ore deposits, ReOs ages for molybdenites agree with RbSr whole-rock ages of host granitoids, while they are 3–12 Ma older than KAr mineral ages of the host rocks. On the other hand, ReOs ages of molybdenites from skarn-, pegmatite-, and greisen-type ore deposits agree with KAr ages of the host rocks. The comparison of ReOs ages for molybdenites with RbSr and KAr ages of host rocks in vein-type deposits suggests that ReOs closure temperature for vein molybdenite is close to whole-rock RbSr closure temperature for host granitic rocks, i.e., roughly estimated to be around 500°C, and higher than KAr closure temperature for host granodiorites, i.e., 230–370°C, if the thermal history of vein molybdenites is essentially equivalent to that of host rocks. One possible explanation for age discrepancies of 3–12 Ma between ReOs ages for molybdenites and KAr ages for the wall rocks, observed in vein-type molybdenum deposits, is that later thermal events after formation of molybdenum ores may reset KAr ages. Hydrothermal alteration might have occurred in such a temperature condition as to disturb KAr mineral systems in wall rocks and made them younger but not the ReOs system in molybdenite. The other explanation might be that the age discrepancies correspond to a cooling interval from ca. 500 to 300°C of host granitoid. On the other hand, the agreement of ReOs ages of molybdenites and KAr ages of host rocks in skarn-, pegmatite-, and greisen-type deposits in this study indicates no later disturbance for KAr system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits.