NOVELTY - A modification system (1) comprises modifier (2) which has modification catalyst (3) which modifies hydrogen type fuel and produces hydrogen, and oxidation catalyst (4) which is positioned by upstream and oxidizes by oxygen environment rather than modification catalyst, reducing environment holding unit which holds carrier of cold-starting catalyst (5) in reduced state over the suspension period of the modifier, and 1st upstream path which has 1st branch part used as upstream edge part and 2nd branch part positioned more downstream than the 1st branch part.    USE - A modification system (claimed).    ADVANTAGE - Arrival time to the modification possible temperature of modification catalyst is shortened. Power consumption is outstanding.    DETAILED DESCRIPTION - A modification system (1) comprises modifier (2) which has modification catalyst (3) which modifies hydrogen type fuel and produces hydrogen, and oxidation catalyst (4) which is positioned by upstream and oxidizes by oxygen environment rather than modification catalyst, reducing environment holding unit which holds carrier of cold-starting catalyst (5) in reduced state over the suspension period of the modifier, 1st upstream path which has 1st branch part used as upstream edge part and 2nd branch part positioned more downstream than the 1st branch part, 2nd upstream path which is formed to branch from 1st upstream path in 1st branch part, 1st fuel-injection apparatus which supplies hydrogen type fuel to modifier through 2nd upstream path, 1st control valve which controls inflow of air to path of 1st upstream path and 2nd upstream path, and 2nd control valve which controls fluid connection with 2nd upstream path and 1st upstream path in 2nd branch part. The reducing environment holding unit controls 1st and 2nd control valves and 1st fuel-injection apparatus so that residual gas that remains for cold-starting catalyst may be pushed out and cold-starting catalyst may be filled by hydrogen type fuel.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the whole structure of the modification system.    Modification system (1)    Modifier (2)    Modification catalyst (3)    Oxidation catalyst (4)    Cold-starting catalyst (5)