PURPOSE: To reduce the abrasion and breakage of a bit and efficiently do a casing excavation by controlling the pressing force applied to a casing head in a casing excavator.

CONSTITUTION: An oil path (e) is branched between the expansion side and shrinkage side oil paths (c), (d) of a lift cylinder 15 fitted to a casing excavator, and a check valve 6, a relief valve 11, and a throttle valve 8 are arranged in the oil path (e) via a solenoid valve 7. The expansion side pressure of the lift cylinder 15 is controlled, the controlled oil is utilized as the supply oil to the shrinkage side (rod side), and the excess oil is returned to a tank T.