A method of manufacturing a vehicle body frame of the present invention is a method of manufacturing a vehicle body frame including a roof structure, side structures that face each other, and a floor structure. The method includes a primary joining step of butt-joining only one set of face plates among the inner face plates and outer face plates of the plurality of panel boards that form the side structures, and manufacturing intermediate bodies of the side structures; a structure joining step of joining the roof structure, the floor structure, and the intermediate bodies; and a secondary joining step of joining, after the structure joining step, the other set of face plates among the inner face plates and the outer face plates of the plurality of panel boards that form the intermediate bodies to complete the side structures.