A combustion apparatus includes a burner having a sheet-metal combustion plate which covers an open surface of a burner body; and a combustion box having a connection flange part to be coupled to a body flange part enclosing the open surface of the burner body, and is equipped with a cooling means for cooling the combustion box, an arrangement is made that overheating of the burner body due to heat transmission from the combustion plate can be prevented, even without enhancing thermal insulation performance of a packing to be interposed between a combustion plate flange part and the body flange part. The connection flange part is kept in direct contact with the combustion plate flange part and, coupled to the body flange part. The packing is provided with: a combustion plate seal part to be pressed under pressure against the combustion plate flange part; and a combustion box seal.