A vehicular underframe structure may an include a lower panel including a composite material and having a central portion curved upwards along a longitudinal direction thereof; an upper panel including a composite material and combined on the lower panel wherein an undersurface thereof faces an upper surface of the lower panel with an internal compartment formed therebetween; and a rib portion provided in the internal compartment and having a lower rib integrally protruding upwards from the upper surface of the lower panel and an upper rib integrally protruding downwards from the undersurface of the upper panel.