PURPOSE: To efficiently obtain a superfine particulate powder having a surface solid strong acid point without calcination by bringing the vapor of a specified silicone compd. into contact with an amorphous superfine particulate powder.

CONSTITUTION: The vapor of at least one kind among the following silicone compds. is brought into contact with an amorphous superfine particulate powder. The silicone compds. are expressed by the general formula (R1HSiO)a(R2R3SiO)b(R4R5R6SiO1/2)c. In the formula, R1, R2 and R3 are independently hydrogen atom, a 1-10C hydrocarbonic group or a 1-10C hydrocarbonic group substituted for ≥1 halogen atoms except that R1, R2 and R3 are simultaneously hydrogen atom, R4, R5 and R6 are independently hydrogen atom, a 1-10C hydrocarbonic group or a hydrocarbonic group substituted for ≥1 hydrogen atoms, (a) is an integer of 0 or ≥1, (b) is an integer of 0 or ≥1 and (c) is 0 or 2 where the sum of (a) and (b) is an integer of ≥3 when (c) is 0.