Abstract: An understructure including a dished form of floor panel and an angled dashboard panel secured as by welding to the front side of the dished floor panel to define therebetween a confined hollow space extending transversely of the automotive body. Trough-like side frames are each secured to the adjacent side of the dished floor panel to define thereon a confined hollow space. Even if open gaps remain in the welded joints between the members described, any water possibly rising through such open gaps into the hollow spaces is collected therein and never enters the car room. Sound propagation through the open gaps into the car room is also minimized. Side frames extending forwardly from the dashboard panel and floor frames secured to the underside of the floor panel and integrally joined with the respective side frames make the understructure extraordinarily high in rigidity in cooperation with the hollow spaces formed on the floor panel and, among others, with the transversely extending one.