A reforming device (10) according to the present invention has a compressor (11), a first heat exchanger (12), a desulfurization device (13), a reformer (14), a raw material gas branching line (L11) that extracts a compressed natural gas (21) from a downstream side of the desulfurization device (13) with respect to the flow direction of the natural gas (21) and supplies the natural gas (21) to the reformer (14), and a flue gas discharging line (L12) that discharges a flue gas (22) generated in the reformer (14), wherein the first heat exchanger (12) is provided in the flue gas discharging line (L12), and the flue gas (22) is used as a heating medium of the compressed natural gas (21).