To manufacture a lightweight electronic apparatus casing with excellent cooling efficiency at a low cost.

The electronic apparatus casing comprises a radiation fin 2 formed of a material excellent in a radiation effect; and a lightweight Mg alloy-made casing 1 in which the radiation fin previously arranged in a casing metal mold is integrally formed by injection molding, and the electronic apparatus casing is manufactured. A fitting detent 3 is provided in the region of a joining plane of a casing main body of the radiation fin 2, and when a Mg alloy is filled in the casing metal mold by thixo-molding, the fitting detent 3 is deformed under the filling pressure of the Mg alloy, to raise fitting strength between the radiation fin 2 and the Mg alloy-made casing 1.