PURPOSE: To improve cleaning efficiency by installing a seal type high-pressure washing automatic apparatus on a reaction vessel and cleaning the reaction vessel in a method in which a lance having a cleaning nozzle unit at its top moves vertically keeping inclination from the axial direction of the reaction vessel and the vertical direction.

CONSTITUTION: A cleaning apparatus 3 is installed on a reaction vessel 1 through a gate valve 2, and a lance 5 having a cleaning nozzle unit 4 at its top is inclined by 30° to 90° from the axial direction of the reaction vessel 1 and by 10° to 45° from the vertical direction. The housing box 6 of the cleaning nozzle unit 4 is installed on the gate valve 2 by fixing it on a support base 17. The lance is supported by supporting parts 10, 11 consisting of three rollers, each fixed at intervals of 120°, to be able to move vertically by the rotation of the rollers. In this composition, after the reaction, a nitrogen gas is supplied to seals 8, 9, the gate valve 2 is opened, the lance 5 is inserted into the reaction vessel 1 by cylinders 13, 14, and the reaction vessel 1 is cleaned with high- pressure water ejected from the cleaning nozzle unit 4.