The melting of plutonium and uranium mixed oxide containing Pu of more than 30% was investigated using a tungsten capsule and a rhenium inner capsule. In the conventional measurements of (Pu, U)O2.00 in the tungsten capsule, a liquid phase of tungsten and plutonium oxide appeared in the mixed oxide during melting. This liquid phase was found to have an effect on the measurement of melting point. Therefore, the rhenium inner capsule was used to avoid the effect. The solidus and liquidus temperatures in the UO2-PuO2 system were decided from the (Pu, U)O2.00 data measured using the rhenium capsule, and the effect of the Am content on the solidus temperature was evaluated. The variation of the solidus and liquidus temperatures in the UO2-PuO2-AmO2 ternary system was represented to an accuracy of σ = ±9 K and σ = ±16 K, respectively, by the ideal solution model.