The present invention utilizes a permanent steel-PC composite beam as a construction load support in the reverse drilling method, while omitting or minimizing the installation of temporary supports, while supporting the unsupported reverse drilling method, which can reduce the height of the floor through distributed support of the permanent steel-PC composite beam. The present invention relates to a steel-PC composite beam that can be preferably applied to the reverse drilling method.;The layer-reduced unsupported reverse drilling method using the permanent steel-PC composite beam according to the present invention as a construction load support includes the steps of: (a) constructing an earth wall; (b) constructing the inner pillar; (c) constructing a one-layer interlayer structure while tacking; (d) excavating; (e1) Steel-PC composite beams prepared by combining steel and PC are installed so as to connect neighboring inner pillars at a lower construction location, and the steel-PC composite beams are formed by using a first suspension material. Installing to hang on; (f) installing the slab bottom plate and reinforcing the slab reinforcement, and then casting the concrete to construct the interlayer structure in which the steel-PC composite beam and the slab are integrated; (g) Repeating the steps (d) to (f), step (e1) is to the interlayer structure of one or more construction completed layer including the construction completed layer directly above the layer to be constructed by steel-PC composite beam It is characterized in that it comprises a;;PC composite beam, unsupported, reverse punching method, suspension
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