To reduce working air pressure by excavating a vertical hole functioning as a deep well and a muck pit before a pneumatic caisson case body is constructed.

Before a caisson case body is constructed, a preceding pit 14 which is a vertical pit is excavated. The cutting edge fitting of the caisson is installed, form for tunnel lining is constructed and outfitting of a shaft and a lock is performed. A ceiling running type caisson shovel 15 is mounted in a working room 16 of the caisson case body, a normal soil layer is excavated with the caisson shovel 15, and the caisson case body is settled. A submerged pump 24 is set in a preceding pit 14 in which ground water entering the working room 16 is allowed to flow, and ground water is drained through a drain pipe 25. Only height of a soil discharge bucket is always excavated so that blasting muck 27 in the preceding pit 14 may naturally enter the soil discharge bucket 22 hung from a shaft 26. Thus, a ground water level can be lowered, working air pressure can be reduced and a working time can be secured long.