A reinforcement structure that reinforces a boom of a work machine having a top plate, a pair of side plates, and a bottom plate forming an inner space therebetween. The plates are configured to form a curved portion at the middle of the boom along the longitudinal direction. The structure comprises a baffle disposed inside the boom rearward of the curved portion across the inner space. The baffle is a plate laterally bent so as to have a convex shape extending forward in the longitudinal direction. The reinforcement structure helps prevent twisting of the boom while dispersing stress concentrations in rear upper portions of the side plates compare to conventional flat baffle plates.