NOVELTY - A fuel cell fuel reformer (1) comprises gas reactor (10), fluid tubes (12), external container (20), and sealant each having thermal expansion coefficients, with one of the coefficients being a maximum and another one being a minimum relative to the other coefficients. The maximum thermal expansion coefficient is no greater than ten times the minimum thermal expansion coefficient.    USE - The reformer is used for fuel cell apparatus for reforming fuel, e.g. aqueous methanol (claimed) or natural gas, into hydrogen rich gas (claimed).    ADVANTAGE - The size of the fuel reformer may be reduced while lowering the energy loss caused by heat conduction due to convection of gas such as air.    DETAILED DESCRIPTION - The fuel cell fuel reformer comprises:    (a) gas reactor for reforming non-reformed fuel to produce reformed fuel;    (b) electric heater that heats the gas reactor;    (c) external container including an inner vacuum chamber (20a) that accommodates the gas reactor and the electric heater, the external container further including multiple receiving portions;    (d) electric wire extending through the external container to supply electric power to the electric heater in the inner vacuum chamber;    (e) fluid tubes respectively received by the receiving portions and extending through the external container, at least one of the fluid tubes supplying the non-reformed fuel to the gas reactor in the inner vacuum chamber, and at least another one of the fluid tubes discharging the reformed fuel produced by the gas reactor; and    (f) sealant that seals gaps between the fluid tubes and the receiving portions; where the gas reactor, the fluid tubes, the external container, and the sealant each have thermal expansion coefficients, with one of the coefficients being a maximum and another one being a minimum relative to the other coefficients, and the maximum one of the thermal expansion coefficients being no greater than ten times the minimum one of the thermal expansion coefficients.    The thermal expansion coefficient of the gas reactor is 20 x 10-7 - 50 x 10-7 /degreesC; the difference between the thermal expansion coefficient of the external container and the thermal expansion coefficient of the gas reactor is plus minus 10 x l0-7 /degreesC; and the difference between the thermal expansion coefficient of the fluid tubes and the thermal expansion coefficient of the gas reactor and the difference between the thermal expansion coefficient of the sealant and the thermal expansion coefficient of the gas reactor are both plus minus 15 x 10-7 /degreesC.    DESCRIPTION OF DRAWING(S) - The figure is a cross-sectional view showing a fuel reformer for a portable fuel cell apparatus.    Fuel cell fuel reformer (1)    Gas reactor (10)    Fluid tubes (12)    External container (20)    Vacuum chamber (20a)