To provide an HIP-treating method for light alloy casting with fused slat and the fused slat for HIP-treatment which hardly develops the corrosion of an apparatus and hardly reacts with the light alloy casting.

The HIP-treatment for light alloy casting, is performed by using the fused salt having the composition of 40-50% sodium nitrite, 40-50% potassium nitrate and 1-10% sodium nitrate, and 140-200°C melting point as a heat and a pressure mediums. Further, before performing the above HIP- treatment, this light alloy casting is preheated with the salt bath using this fused salt.