To correctly produce pressure drag that is resistive against pressures including earth pressure, water pressure and, etc., in a cutter chamber, and to improve efficiency of motive power.

Two earth-carrying screw blades 13 and 14 are rotatively arranged in cylindrical casings 11 and 12 having their front ends opened to a cutter chamber 4 while having their rear ends opened to an atmospheric pressure chamber. On driving the screw blades, discharge efficiency is obtained from the pressure inside the cutter chamber 4 and excavation speed of a shield machine body 2, and rotational frequencies of the screw blades 13 and 14 are controlled by a controller 19 so that the discharge efficiency becomes 0.7 or more and discharge efficiency on discharging by a front conveyer part 7A becomes equal to or larger than the discharge efficiency on discharging by a rear conveyer 7B. The pressure inside the cylindrical casings on a position near an earth discharging port 12a is controlled to become approximately the same as atmospheric pressure.