Abstract: In a lid member having a long and short edge plate portions which are adjacent to each other, and approximately formed as an L-shaped form in its cross section, the long edge plate portion has a plurality of first groove portions which extend approximately in parallel to a ridge line portion which is held in common with the short edge plate portion adjacent in a width direction so as to be arranged side by side while being spaced from each other, and are formed so as to be concaved to a back side in a thickness direction in relation to an upper surface, and a plurality of second groove portions which are formed so as to be concaved to the back side in the thickness direction in relation to the upper surface, and are connected both end portions in a length direction respectively to a plurality of first groove portions.