To reduce the thrust of feed mechanism when the rotary resistance of a crawler drill increases during drilling.

An advancing pipe line FF and a backing pipe line FRE in a feed mechanism 4 are connected thereto with a fast feed solenoid selector valve 9, and further, the advancing pipe line FF is connected thereto with a pressure reducing valve 6 downstream of the solenoid valve 9, and an advancing flow regulating valve 10 in parallel with the solenoid valve 9 while the backing pipe line FR is connected thereto with a relief valve 8 for regulating the backing pressure, upstream of the solenoid valve 9, and a backing pressure regulating valve 12 in parallel with the solenoid valve 9. Next, a relief valve 15 for regulating the advancing pressure is connected to a working pressure control port of the pressure reducing valve 6 through an advancing pressure selector valve 14, and a pilot selector valve 23 and a check valve 24 are connected between the backing flow regulating valve 12 and the feed mechanism 4. Further, the backing pipe line FR and the advancing pipe line FF are connected together through the pilot selector valve 23, and an orifice 25 and a check valve 26 are connected thereto in parallel. A pilot pipe line P from the pilot selector valve 23 is connected to a normal rotation pipe line RF through a pilot changing solenoid valve 27, and is connected to a tank 22 through an orifice 28.