To high-efficiently and rapidly dig a drainage ditch at the end part of a balcony and a slab, such as a corridor, fronting on the outdoor place.

In PC forms 1, the edge of a bottom 11 thereof is brought into abutment against the lower end of inner forms 23 for a straight wall by which the inner side of the straight wall is built and the PC forms are installed in a floating state on the lower forms 21 for a slab through a plurality of support members 5 between which a given distance is provided. The PC forms 1 are built integrally with concrete with which a concreting space between above the lower forms 21 for a slab and a concreting space between the outer forms 22 for a straight wall and the inner forms 23 for a straight wall are continuously filled. The drainage ditch is built by means of the PC forms 1 themselves.