A method of manufacturing a closed cross-section structural member is disclosed. The closed cross-section structural member includes: a first panel and a second panel joined to each other to form a closed cross-section; and a fiber reinforced plastic (FRP) material bonded to the first panel. The method includes: attaching the FRP material to the first panel with an adhesive; curing the FRP material and the adhesive by heating the FRP material and the adhesive together with the first panel; and joining the second panel to the first panel in which the FRP material and the adhesive are cured.