Disclosed is an automobile underbody structure, wherein a pair of laterally-spaced floor frames 10 are disposed to extend longitudinally in such a manner as to interpose a tunnel region 6 of a front floor portion 2 therebetween and joined to a bottom surface of the front floor portion 2, and a cross member 15, 16 is disposed to extend laterally so as to connect each side-sill 7 and the tunnel region 6 and joined to a top surface of the front floor portion 2. Each of the floor frames 10 is gradually inclined laterally inward toward the rear end thereof so as to be obliquely disposed relative to the cross member 15, 16 in top plan view. A rearward displacement of the floor frames 10 during a head-on collision exerts a compressive force on a region of the cross member 15, 16 located on the laterally outward side relative to each of the floor frames 10, and a tensile force on a region of the cross member 15, 16 located on the laterally inward side relative to each of the floor frames 10. The automobile underbody structure of the present invention makes it possible to effectively receive a shock during a head-on collision by the cross member.