To provide a fastening structure and a fastening method of a cab mount bracket for easily performing both fastening works without checking one fastening work by a fastening member for the other fastening work, even if a place for fastening the cab mount bracket is close to a place for fastening an engine mount.

The cab mount bracket 3 is fastened to a frame 1 upper part, and the engine mount 5 is fastened in the vicinity of the fastening position of the cab mount bracket 3, and the cab mount bracket 3 is placed on the upper surface of a flange part 11, and is fastened to the flange part 11 with a plurality of fastening bolts B11 extending by penetrating through the flange part 11 and a plate nut 6 threadedly engaging with the bolts.