To use a cooling oil bath having the most suitable properties by treating an oil layer remained on particles with a lighter oil having a relatively low viscosity and boiling temperature when magnesium alloy particles and the like are produced from molten metal, cooled with a cooling oil, centrifuged and succeedingly heated.

The magnesium particles containing oil are supplied to a supply funnel 4 rotating at the same speed as that of a basket through an inlet pipe 3, and almost all liquid is discharged through a sieve opening 5 of a supply zone in the basket to the outside of a centrifugal separator. At the first chamber a mother liquid is separated from the starting magnesium particles and discharged, and at the second chamber a specific amount of lighter oil is sprayed to the magnesium particles remaining an oil layer of the first cooling oil. The lighter oil is supplied from a pipe 6, and by selecting a kind and an amount of the lighter oil, properties of the final oil layer remained on the particle are controlled.