PURPOSE: To remove depositions on a material to be washed immersed in high temperature and high pressure washing liquid by putting in a pressure vessel the material to be washed and water or washing solution where water is mixed with a water detergent, heating the washing liquid and keeping the pressure in the pressure vessel and the temperature of the washing solution at a specified pressure and a specified temperature respectively.

CONSTITUTION: In a pressure vessel 1 are put a material to be washed and water or washing solution where water is mixed with a water detergent and the washing liquid is heated to keep the pressure in the pressure vessel at 0.5-3kg/cm2 and to keep the temperature of the washing liquid at 80-130°C at which the washing liquid is not substantially vaporized at the internal pressure of the pressure vessel and the depositions on time material to be washed immersed in the high temperature and high pressure washing liquid are washed with it. Thus, oil, etc., stuck to substances, such as metal products are effectively removed practically with water and simultaneously the dissipation of the detergent in the air is prevented.