PURPOSE: To effectively excavate an expanded bottom by a method in which a switch valve is provided between an oil-pressure cylinder to open or close a bottom-expanding blade and an oil-pressure cylinder to open or close a rotary excavating blade, and vertically uprising excavation is made for expanded bottom.

CONSTITUTION: Vertically uprising excavation for expanded bottom is made by a tip bit 11 and a rotary excavating blade 12. A switch valve 24 for switching the supply and discharge of pressure oil for an oil-pressure cylinder 20 is provided between an oil-pressure cylinder 16 to open or close a bottom- expanding blade 10 and the oil-pressure cylinder 20 to open or close the blade 12. Only the cylinder 16 can be operated by closing a valve 33 in the oil-pressure circuit. The excavating efficiency can thus be raised, and the trouble of shortage in rigidity to excavating torque can also be obviated.