The invention provides a furnace grate structural body which can correspond to high temperature combustion with lowering harmful substances and high efficiency heat recovery as targets. The furnace grate structural body is used for a mechanical stoker combustion device and comprises a first furnace grate and a second furnace grate, wherein the first furnace grate is provided with an upper wall part and a front wall part which extends from the front end part of the upper wall part; the second furnace grate is adjacent to the first furnace grate in the width direction; the first furnace grate is provided with a convex surface part on the side surface which is adjacent to the second furnace grate; the convex surface part of the first furnace grate is abutted to the side surface of the second furnace grate; a first slit is formed between the side surface of the first furnace grate and the side surface of the second furnace grate; and the first furnace grate is mutually connected with the second furnace grate under the state that the convex surface part of the first furnace grate is abutted to the side surface of the second furnace grate.