To provide an eaves soffit plate joining member, a construction method of the eaves soffit plate joining member and a building using the eaves soffit plate joining member, easy in installation in a clearance between mutual eaves soffit plates, and superior in installation workability, without interfering with a peripheral member in installation.

This eaves soffit plate joining member CJ is provided with a plate-like web part W interposed along the clearance, a pair of flange parts UF and LF mutually extending in parallel in the direction along an eaves soffit plate CP2 from both upper-lower edges of the web part W, and a cover flange part MF extending in the direction along an eaves soffit plate CP1 on the opposite side of the pair of flange parts UF and LF to the web part W from the lower edge of the web part W. The clearance is covered with the cover flange part MF, by adjacently sticking an end part to the eaves soffit plate CP1 stuck in advance to the eaves reverse side, by being installed on the eaves soffit plate CP2 by sandwiching the clearance side end part of the eaves soffit plate CP2 by the pair of flange parts UF and LF.