To provide a post-beam joining tecknique which is excellent in aseismatic performance, and has high strength and reliability.

This post-beam joint structure is formed by a plate-like joining member 11a which can be mounted to cross a post member 10, and a beam connecting part 11b projected on the outer periphery of the joining member 11a, a diaphragm 11 having a V-shaped beveling for joining a flange 13a of a beam member 13 by welding formed at the tip of the beam connecting part 11b is mounted at the same distance as the flange space of the beam member 13 on the post member 10, and the flange 13a end part of the beam member 13 is joined to the tip of the beam connecting part 11b of the diaphragm 11 by welding.