PURPOSE: To produce a partially reinforced composite material using an Mg-base metal as a matrix by arranging a preform constituted of a reinforcing material by a ceramics material and a specified infiltrating material into a mold and casting the molten material of an Mg-base metal therein.

CONSTITUTION: A reinforcing material 2 which does not reacted with a molten Mg-base metal such as SiC, Al2O3 and TiC having a granular shape with 0.1 to 100μm particle diameter or a whisker or fibrous shape is mixed with an infiltrating assistant 2 of SiO2, ZnO or the like having 1 to 50% volume ratio, which is formed into a preform 3 having a specific shape. This is preheated to 600 to 700°C and is set in a die previously heated to 200 to 500°C, and the molten material 5 of pure Mg or an Mg alloy is cast therein from a runner 4a. The molten material 5 of the Mg or Mg alloy is brought into contact with the preform 3 and is infiltrated into the preform 3 while exothermic reaction is caused with the infiltrating assistant 2, by which the Mg-base partially reinforced composite member 6 compoundable with the preform 3 without needing pressurizing and using ceramics as a reinforcing composite material can be obtd.