A reinforcement structure comprising: an adherend and a reinforcement sheet adhering to the adherend and reinforcing the adherend, wherein the adherend includes a first surface extending in a first direction, a second surface extending in a second direction crossing the first direction, and a first corner portion connecting the first surface to the second surface; the reinforcement sheet includes a front layer containing a plurality of fibers and positioned at spaced intervals to the first surface, the second surface, and the first corner portion and a core material layer containing a resin and disposed between the front layer and the adherend; the front layer includes a first front layer portion positioned at spaced intervals to the first surface in the second direction and extending in the first direction, a second front layer portion positioned at spaced intervals to the second surface in the first direction and extending in the second direction, and a third front layer portion positioned at spaced intervals to the first corner portion in a third direction crossing both directions of the first direction and the second direction and connecting the first front layer portion to the second front layer portion; and a gap between the first surface and the third front layer portion in the second direction gradually increases as it gets closer to the first corner portion in the first direction with respect to a gap between the first surface and the first front layer portion in the second direction, and a gap between the second surface and the third front layer portion in the first direction gradually increases as it gets closer to the first corner portion in the second direction with respect to a gap between the second surface and the second front layer portion in the first direction. an adherend and a reinforcement sheet adhering to the adherend and reinforcing the adherend, wherein the adherend includes a first surface extending in a first direction, a second surface extending in a second direction crossing the first direction, and a first corner portion connecting the first surface to the second surface; the reinforcement sheet includes a front layer containing a plurality of fibers and positioned at spaced intervals to the first surface, the second surface, and the first corner portion and a core material layer containing a resin and disposed between the front layer and the adherend; the front layer includes a first front layer portion positioned at spaced intervals to the first surface in the second direction and extending in the first direction, a second front layer portion positioned at spaced intervals to the second surface in the first direction and extending in the second direction, and a third front layer portion positioned at spaced intervals to the first corner portion in a third direction crossing both directions of the first direction and the second direction and connecting the first front layer portion to the second front layer portion; and a gap between the first surface and the third front layer portion in the second direction gradually increases as it gets closer to the first corner portion in the first direction with respect to a gap between the first surface and the first front layer portion in the second direction, and a gap between the second surface and the third front layer portion in the first direction gradually increases as it gets closer to the first corner portion in the second direction with respect to a gap between the second surface and the second front layer portion in the first direction.