NOVELTY - A modifier system (10) comprises modifier (2) which generates reformed gas (G3) used as fuel of solid oxide fuel cell (6) by making unmodified gas (G2) which is mixed gas of raw fuel gas (G1) and steam (S1) contact a modification catalyst, raw fuel gas supply part (3) to supply raw fuel gas to modifier, steam supply part (4) to supply water vapor to modifier, control part (5) which controls raw fuel gas supply part and steam supply part, 1st detection part (7) to detect temperature or pressure of unmodified gas, and 2nd detection part (8) to detect temperature or pressure of reformed gas.    USE - A modifier system for fuel cell system (claimed).    ADVANTAGE - Modifier system operates with high efficiency.    DETAILED DESCRIPTION - A modifier system (10) comprises modifier (2) which generates reformed gas (G3) used as fuel of solid oxide fuel cell (6) by making unmodified gas (G2) which is mixed gas of raw fuel gas (G1) and steam (S1) contact a modification catalyst, raw fuel gas supply part (3) to supply raw fuel gas to modifier, steam supply part (4) to supply water vapor to modifier, control part (5) which controls raw fuel gas supply part and steam supply part, 1st detection part (7) to detect temperature or pressure of unmodified gas, and 2nd detection part (8) to detect temperature or pressure of reformed gas. Based on temperature or pressure detected by 1st and 2nd detection parts, when temperature difference or pressure difference between unmodified gas and reformed gas is in a predetermined range, ratio S/C of amount (S) of steam supplied to modifier and amount (C) of raw fuel gas supplied to modifier becomes a normal value, and when temperature difference or pressure difference is outside the predetermined range, ratio S/C becomes an elevated fixed value higher than normal value. INDEPENDENT CLAIMS are included for:    (1) fuel cell system (1); and    (2) operating method of fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a fuel cell system.    Fuel cell system (1)    Modifier (2)    Raw fuel gas supply part (3)    Steam supply part (4)    Control part (5)    Solid oxide fuel cell (6)    1st detection part (7)    2nd detection part (8)    Raw fuel gas (G1)    Unmodified gas (G2)    Reformed gas (G3)    Steam (S1)