Screw conveyors are generally used as excavated soil dischargers for earth pressure and muddy earth pressure types of shield machines. The conveyor discharges the soil continuously under loosened earth pressure and underground water pressure held by soil plugs which are formed in the screw grooves and the sand plug zone. In order to clarify the pressure-holding mechanism of the conveyor filled with the soil, the theoretical equations for granular and plasticized materials which represent the pressure drop in the screw conveyor were formulated. As the result of the investigations, the calculated values were found to be in good agreement with the experimental and practical Ones.