NOVELTY - A modifier (100) comprises evaporation part (104) which evaporates raw materials and liquid hydrocarbon type mineral fuel, modification part which accommodates modification catalyst and modifies raw materials and mineral fuel to high modification fuel of hydrogen content in housing, hydrogen separation membrane which selectively transmits hydrogen in modification fuel, and flow rate limiting mechanism which narrows down flow rate of off gas. The quantity of raw materials and mineral fuel is reduced when pressure of evaporation part and modification part is high.    USE - A modifier for fuel cell system (claimed).    ADVANTAGE - Steadily and continuously modifies raw materials and mineral fuel. Has simple structure.    DETAILED DESCRIPTION - A modifier (100) comprises housing (102), evaporation part (104) which evaporates raw materials and liquid hydrocarbon type mineral fuel is provided in housing, modification part which accommodates modification catalyst that is arranged on evaporation part, connects with evaporation part and modifies raw materials and mineral fuel to high modification fuel of hydrogen content in housing, hydrogen separation membrane which selectively transmits hydrogen in modification fuel, hydrogen gas sending opening which sends out hydrogen, off-gas sending opening which sends out off gas, off-gas delivery valve which sends out off gas to space, flow rate limiting mechanism which narrows down flow rate of off gas, and flow path of raw materials and mineral fuel which is formed in straight line at perpendicular direction from bottom part to modification part. The quantity of raw materials and mineral fuel is reduced when pressure of evaporation part and modification part is high. INDEPENDENT CLAIMS are included for:    (1) fuel cell system; and    (2) modification method.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the modifier.    Modifier (100)    Housing (102)    Evaporation part (104)    Air inlet (120)    Fuel cell (300)