PURPOSE: To make improvements in machining a work, by detecting a difference in fluid pressure to be received by a pressure receiving body from a machining fluid out of both upper and lower nozzles, while adjusting a position of the pressure receiving body in a way of detecting an impact position directly in an accurate manner according to a pressure balancing degree in an impacting part of the machining fluid.

CONSTITUTION: A pressure receiving part 22 being locked to the tip of a holding part 21 is situated in an impacting part of a machining fluid 5 fed out of both upper and lower nozzles 3 and 4, and receives the machining fluid 5 fed out of both these nozzles 3 and 4 at its upper and lower surfaces, while the holding part 21 is elastically displaced by a pressure differential in the machining fluid 5 out of these nozzles 3 and 4 received by the pressure receiving part 22. With a detector 13 being installed in position between a base end part of the holding part 21 and an arm 17, size and sense in the pressure differential of the machining fluid 5 are detected according to displacement in the holding part 21 and the detected value is displayed on an indicator 23. When the pressure differential is out of the allowable value, a valve 8 is operated as far as necessary, adjusting a flow rate of the machining fluid 5 out of both nozzles 3 and 4, thus a position of the impacting part of the fluid 5 is adjusted right.