A structure including a structural member configured like a frame with four sides is disclosed. A closed cross-section structural member serving as a reinforcing material is provided in a crossing corner of the structural member. The closed cross-section structural member includes two horizontal portions separated from each other in a thickness direction of the structural member and extending in an in-plane direction of the structural member and two vertical portions one of which connects first ends of the two horizontal portions together and the other of which connects second ends of the two horizontal portions together. The closed cross-section structural member includes a first opening formed therein facing an inner-peripheral flat surface of the structural member and a second opening formed therein facing inside of the structural member.