PURPOSE: To reinforce the resisting pressure strength of a frame by forming the longitudinal direction cross section at the side of a gutter shape frame which is joined between the floor back plane and the inner plane of hull in a continuous uneven manner.

CONSTITUTION: A longeron 12 is fayed between a floor 14 and the inner side of a hull 10 as a frame structure. A rib 16 is secured with the longitudinal side plane of the longitudinal 12 to project toward the center of the hull 10 with a certain spacing, the longitudinal directional cross section of the side of the longitudinal 12 being constructed in a continuous uneven manner to reinforce the resisting pressure strength of the longitudinal 12. The edge being joined with the hull 10 of the longitudinal 12, besides, provided with the bending section 17 which is bent somewhat toward the center direction. This enables to prevent from the concentrated stress developed in the hull 10.