Disclosed is a framework connecting device of a prefabricated building structure. Each connection portion of frameworks, which are formed in the shape of a square pipe so as to form columns, rafters or the like, is connected and fixed by front and rear connection pieces and a plurality of bolts and nuts which penetrate the same, wherein, during assembly, the frameworks and the front and rear connection pieces are coupled by indented groves and protrusion portions. Base protrusions of the front and rear connection pieces are supported by the frameworks. One or more support pipes which are respectively connected and fixed by bolts and the nuts are provided between the front and rear connection pieces. Therefore, the assembling work may be readily and easily carried out while the strength of the connection portions is being reinforced.