To produce a preform formed body into which Mg or Mh alloy molten metal is easily infiltrated by dropping a soln. in which reinforcing material powder and infiltrating assistant material powder are dispersed into a preform die, thereafter heating the dispersion soln. and drying it.

Af first, a reinforcing material of ceramic particles not reacted with Mg such as Sin, an infiltrating assistant material such as SiO2 and an organic binder are charged to the inside of a solvent such as water, and stirring is executed to uniformly disperse them. In the case this dispersion soln. 6 is poured into a preform die 7, and filtering is executed, though it depends on the kinds of the reinforcing material and infiltrating assistant material or the like, a peform contg. about 15 to 50vol.% reinforcing material can be formed. In this case, when the reinforcing material and infiltrating assistant material having different precipitating rates in the solvent are used, a concn. gradient can be given to the infiltrating assistant material in the preform formed body, and the infiltrating phenomenon can smoothly be progressed. Then, the preform formed body is heated at the b.p. of the solvent or below and is dried.