Joint structure of cab mounting portion

There is provided a cab structure that a joint portion of a cab and a chassis frame has sufficient shear strength and which can disperse external force to the entire cab or chassis frame to reduce the burden of load and can sufficiently resist the impact load in the longitudinal direction. A front end portion of a cab-under-frame fixed to the outer surface of the floor panel is extended toward a front upper side, and a cab mount bracket is also extended toward a front upper side so that they are integrally joined with a front mounting rail. As a result, these three components are firmly fastened to disperse the incoming force in the longitudinal direction.