A fitting structure is provided for supply of gas and connectable between an air supply tube in the form of a hose or a plastic pipe and an air outlet of an air compressor and includes a fitting and a thermal insulation tube. The fitting includes a first coupling opening connectable with the air outlet of the air compressor and a second coupling opening connectable with the air supply tube. The thermal insulation tube has a first end part fixed inside the second coupling opening of the fitting and a second end part extending outside the second coupling opening such that a portion of the thermal insulation tube that extends outside the second coupling opening is received in the interior of the air supply tube. High-temperature air from the air compressor is conducted through the thermal insulation tube into the air supply tube without being directly applied to the fitting.