A side sill is configured from a side sill outer element comprising an outer upper wall, outer lower wall, an outer side wall, an outer upper flange and an outer lower flange, and from a side sill inner element comprising an inner upper wall, an inner lower wall, an inner side wall, an inner upper flange and an inner lower flange. On the lower end, a pillar has a joint portion held between and joined to the outer upper flange and the inner upper flange, and a first extension part and a second extension part extending from the joint portion into the closed section of the side sill, and includes an extension, the top end of which is joined to the first extension part and/or the second extension part and the bottom end of which is held between and joined to the outer lower flange and the inner lower flange.