Not only a reduction in the load resistant performance of a structural member in a vehicle body having a quenched portion with a tensile strength of 1,470 MPa or greater, but also a reduction in the shock absorbing performance associated with the HAZ softening by welding are suppressed. In a joining structure (10) for a member in a vehicle body in which a first member (11) that is made of steel, has a closed hollow cross-section having no outward flange, and is provided with a quenched portion having a tensile strength of 1,470 MPa or greater, a base material portion having a tensile strength of less than 700 MPa, and a transition portion (17) between the quenched portion (16) and the base material portion (18) in a longitudinal direction thereof, and a second member (12) that is made of steel and overlaps the first member (11) are welded to each other in an overlapping portion, the overlapping portion exists over the base material portion (18) from the quenched portion (16) of the first member (11) through the transition portion (17), and a welding portion (13) formed by welding exists in the transition portion (17) or the base material portion (18) in the first member (11).