To provide a method and a device for discharging mud in a shield tunneling method, which prevent a mud discharge pipe from being clogged with the mud conveyed to a ground-level side and enable smooth discharge of the mud.

This method for discharging the mud comprises: a negative- pressure process wherein the inside, on an end side, of the mud discharge pipe 5 is put in a negative pressure from a position of arrangement of a valve 7, provided on the base-end side of the pipe 5 in a shield machine 1, under a closed state of the valve 7; and a pressurized-air inflow process wherein compressed air is made to flow into the pipe 5 from a vicinity of the valve 7 after the negative-pressure process.