PURPOSE: To make it possible to detect abnormality at the extremely initial stage of destruction, by constituting a capacitor circuit by providing an electrode to the end part of a nozzle and supplying a current to said capacitor circuit.

CONSTITUTION: When high pressure water comes to high pressure water jet having an abrasive material mixed therein to flow out from the leading end of a nozzle 19, a current is supplied to a nozzle abnormality detection circuit to monitor the state of the nozzle 19. The nozzle 19 receives abrasion on the inner wall of the jet passage 26 thereof with the elapse of time and the diameter of said passage 26 is gradually enlarged. When this change is normal, the capacitor constituted of electrodes 30, 31 and the nozzle 19 changes in its capacity at a constant ratio and, therefore, the current value of an ammeter 33 gradually changes. However, when offset abrasion is generated in the passage 26 and partial abrasion markedly advances in the nozzle 19, the current value of the ammeter 3 shows irregular change. Further, when the nozzle 19 is destructed at an area where abrasion advanced excessively, the electrode 30 or 31 is destructed at said area and the electrostatic capacity of the capacitor changes and, therefore, the current value largely changes. By catching this change, the offset abrasion and destruction of the nozzle are detected.