To provide cab underframe structure of a vehicle having strength durable against impact force at the time of collision, etc., capable of absorbing impact energy and performing stable quality ones comparatively at low cost and high in welding strength.

An underframe intermediate part 1 of a cab underframe 8 consisting of an underframe front part 3, the underframe intermediate part 1 and an underframe rear part 4 is constituted of the one made by welding a plural number of closed cross-section frame members. Additionally, their mating parts are fixed by laser welding, etc. Consequently, the underframe intermediate part 1 becomes of high rigidity and the underframe front part 3 is made of the one of comparatively low rigidity in comparison with this, and accordingly, it is structured to carry out strength maintenance of impact force and relative absorption of impact energy.