NOVELTY - A modification system (10) contains modification part (11) which introduces hydrocarbon-based raw material (D) and modification water (W) and modifies raw material to fuel gas (E) containing hydrogen, modification water tank (12) which stores modification water supplied to modification part, modification water line (51) which guides modification water in modification water tank into modification part, and filter (15) where modification water flowing through modification water line passes to forward direction (P) which goes to modification part.    USE - A modification system for fuel cell system (claimed).    ADVANTAGE - Clogging of filter can be prevented, and size of filter can be reduced.    DETAILED DESCRIPTION - A modification system (10) contains modification part (11) which introduces hydrocarbon-based raw material (D) and modification water (W) and modifies raw material to fuel gas (E) containing hydrogen, modification water tank (12) which stores modification water supplied to modification part, modification water line (51) which guides modification water in modification water tank into modification part, filter (15) where modification water flowing through modification water line passes to forward direction (P) which goes to modification part, back-flow unit to make modification water moving in forward direction flow to reverse direction (Q) through filter, and discharge line (59) which discharges modification water flowing in reverse direction from modification water line. An INDEPENDENT CLAIM is included for fuel cell system (1) which contains fuel cell (20) which generates electric power by introduction of oxidizing agent gas (A2) containing oxygen and modification part (11) which produces fuel gas and is positioned in modification system (10).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a fuel cell system.    Fuel cell system (1)    Modification system (10)    Modification part (11)    Water tank (12)    Filter (15)    Fuel cell (20)    Modification water line (51)    Discharge line (59)    Oxidizing agent gas (A2)    Raw material (D)    Fuel gas (E)    Forward direction (P)    Reverse direction (Q)    Modification water (W)