To obtain a diaphragm for joining corrugated steel plate web beams capable of easily providing the corrugated steel plate webs in a short period.

This diaphragm for joining the corrugated steel plate web beams 4 is formed of an upper and a lower diaphragm flange surfaces 2 and diaphragm web surfaces 3 formed in both side surfaces. The diaphragm for joining the corrugated steel plate web beams 4 is provided with a smooth surface for joining ends of the webs 5 of the adjacent corrugated steel plate web beams 4 in the outside surface of the diaphragm web surfaces 3 formed in both the side surfaces. Outside surfaces of the upper and the lower diaphragm flange surfaces 2 are respectively provided with a displacement stopper 6 for preventing displacement of the corrugated steel plate web beams 4 along the longitudinal direction.