PURPOSE: To remarkably accelerate a reductive reaction, by applying ultrasonic waves of a given frequency to a solution containing Fe3+ ions dissolving a metal in the solution for reducing the Fe3+ ions, and activating always the surface of the metal.

CONSTITUTION: A metal is added to a solution containing Fe3+ ions to reduce the Fe3+ ions. In the process, ultrasonic waves of 10W500kHz are applied to the solution in dissolving the metal in the solution. Thus, the surface of the metal is always activated The fresh activated surface is always exposed, and the reductive reaction is remarkably accelerated by the method.