PURPOSE: To remove a stain bonded chemically or diffused in a material to be cleaned in a short time by subjecting the material to ultrasonic cleaning in a prescribed cleaning soln. contg. suspended fine SiO2 particles having a prescribed particle size.

CONSTITUTION: Fine SiO2 particles having ≤50nm particle size are suspended in fuming sulfuric acid, fuming nitric acid or an org. solvent such as acetone or trichlene as a cleaning soln. A material to be cleaned is subjected to ultrasonic cleaning in the cleaning soln. contg. the fine SiO2 particles. The particles hit on the surface of the material and remove the stained layer without deteriorating the surface properties. Thus, the stain bonded chemically or diffused in the material is removed in a short time.