To ensure that a down frame in an off-road motorcycle body frame is lightweight, rigid and easy to weld, a down frame extending downward obliquely and backward from a main frame ahead of an engine is divided along its length into a down frame front portion and a down frame rear portion. Welding joins these portions, which abut each other in the longitudinal direction. The down frame front portion is a pipe wrought product formed by extrusion molding and the down frame rear portion is a casting. Since the down frame is welded along a straight welding line extending along its length, welding is easy. Since a main joint and a lower joint are provided integrally with the down frame rear portion as a casting, a separate gusset can be omitted.