A piping connection unit of a combustion apparatus supplying and discharging air through a duct includes an apparatus-side connecting part, a connecting part and a spacer. The apparatus-side connecting part is to be connected to an air supply part or an exhaust part of the combustion apparatus. The connecting part has a diameter larger than a diameter of the apparatus-side connecting part, and communicates with the apparatus-side connecting part to be connected to a duct to be inserted from an opening part. The spacer is detachably disposed in the connecting part. The spacer is disposed between an inner wall of the connecting part and the duct to be inserted when the duct has a pipe diameter smaller than an inner diameter of the connecting part. The spacer is removed from the connecting part when the duct has a pipe diameter equivalent to the inner diameter of the connecting part.