PURPOSE: To easily detect refuse adhering to a fixed screen always in navigation in a water jet propulsion machine.

CONSTITUTION: A waterjet propulsion machine, for obtaining thrust by leading water into a pump casing 23 from an intake 22 opened in a bottom 21 to pressurize the water by an impeller 25 in this pump casing 23 to jet the water from a nozzle 27 provided on a stern, is provided with a drainboard-shape fixed screen 31 wherein plural flat plates 30 are juxtaposed in the longitudinal direction of the intake 22, a vibration detecting means arranged in the plates 30 to detect the vibration of them, and a signal processing means for discriminating the existence of refuse adhering to the plates 30 based on the detection result of this vibration detecting means.