A first bracket is attached to a heat source apparatus. A second bracket is fixed to a wall. The second bracket has an engagement portion over which the first bracket is hooked. The first bracket has a through hole through which the engagement portion is inserted. The first bracket has a plurality of protruding portions that protrude from an upper end of the through hole into the through hole. The plurality of protruding portions are arranged side by side along the upper end of the through hole.