PURPOSE: To supply operating fluid to a hydraulic cylinder and to promote work efficiency by switching over a solenoid valve at optimum switching timing with a controller in accordance with detected value of a piston position of the hydraulic cylinder and rock information.

CONSTITUTION: An electric output is outputted to a solenoid 21A from a controller 22 at optimum switching timing in accordance with a program set in advance based on detected value of a position detector 27 and rock information. After that, a solenoid valve 21 is placed in a right side position shown by a figure, operating fluid is supplied to oil chambers 29 and 30 of a hydraulic cylinder 20, and a piston rod 26 is raised with difference between projected net areas of pistons 24 and 25. Then, upper dead point arrival is detected by the position detector 27, and it is inputted to the controller 22 to output the electric output to the solenoid 21A at optimum switching timing for a rock. Successively, a selector valve 21 is switched over to a left side position, the operation fluid in the oil chamber 29 is supplied to the inside of an oil tank 11 through the selector valve 21. According to the constitution, the piston rod 26 is lowered to hit the rock, and it is crushed to bore a hole.