To provide a joint structure that can improve strength in a side member joint portion by permitting spot welding.

In this joint structure, a side member 2 is composed of a front member 10 having a generally hat-shaped cross section with flanges 15b, 16b, and a rear member 11 with flanges 11a, 11c, in such a way that the front member 10 and the rear member 11 are jointed with lapping the flanges 15b and 11a, and 16b and 11c, respectively. Further, a first and a second reinforcements 20, 21 having generally hat-shaped cross sections with flanges 20a, 20c, 21a, 20c are formed at the joints of the front and rear members 10, 11. Spot weld portions S1 to S5 made of two or three layers are configured by providing notches 11b, 20b, 21b to one or two flanges out of the four layers. The four layers are configured by lapping the flanges 20a, 21a on the flanges 15b, 11a, or the flanges 20c, 21c on the flanges 16b, 11c.