PURPOSE: To permit the use of an inexpensive and lightweight hose, by providing a detector for sensing the opening and closing condition of a switch clock and a control means for controlling a pump-driving engine upon reception of a signal from the detector.

CONSTITUTION: To spray agricultural chemical, they are placed in a receiving tank 1 and a switch clock 11 is then closed. In doing so, an engine 4 is left idling after start, because the switch cock 11 is closed and a solenoid is not energized. When an operator opens the switch cock 11, while pulling out a hose 8, a detector 13 generates 'on' signal which is inputted in a control circuit through a box 25 and a control box 24 to energize the solenoid. This keeps the engine 4 running over a normal speed range, whereby a pump 3 is driven and the agricultural chemicals are discharged from a nozzle 10 through the hose 8. To stop the spray operation, the operator can close the switch cock 11 and then the detector 13 generates 'off' signal to stop the operation of the pump 3 through the same circuit as with the 'on' signal.