PURPOSE: To reduce reaction force at high pressure injection of water or diluted detergent to remove astonishment or fear feeling of an operator by providing a pressure control means which increases injection pressure step by step in the early term of injection and decreases injection pressure step by step in the latter term.

CONSTITUTION: Injection pressure from a nozzle 3a is increased step by step in the early term of injection in a spray type carwash work, and decreases the pressure step by step in the latter term of injection, by opening a solenoid valve EV3 on a bypass circuit L3. Hereby, reaction at high pressure injection of water from the nozzle 3a can be reduced.