NOVELTY - Fuel tank comprises pressure tank including a space; dividing portion dividing the space into spaces so that when one of the spaces is increased in volume, the other of the spaces is decreased in volume, where one space is configured to accommodate dimethyl ether and the other space is configured to accommodate water; dimethyl ether outlet; and water outlet.    USE - The fuel tank is used for a hydrogen gas generator and a fuel cell system (claimed).    ADVANTAGE - Fuel is less varied in rate of flow and composition. The hydrogen gas generator has an extended in service life.    DETAILED DESCRIPTION - Fuel tank (101) comprises:    (a) pressure tank (102) including a space configured to accommodate dimethyl ether and water;    (b) dividing portion (103) dividing the space into spaces so that when one of the spaces is increased in volume, the other of the spaces is decreased in volume and so that the one of the spaces is configured to accommodate dimethyl ether (13) and the other of the spaces is configured to accommodate water (14);    (c) first outlet (15a) provided on the pressure tank so that the one of the spaces can be opened and dimethyl ether taken out; and    (d) second outlet (16a) provided on the pressure tank so that the other of the spaces can be opened and water taken out.    INDEPENDENT CLAIMS are also included for:    (i) hydrogen gas generator comprising a pressure tank; dividing portion; first outlet provided on the pressure tank; second outlet provided on the pressure tank; reforming portion configured to use dimethyl ether and water to obtain a reformed gas containing hydrogen with carbon monoxide (CO); and CO removing portion that is configured to remove at least a part of the CO contained in the reformed gas; and    (ii) fuel cell system comprising a pressure tank; a dividing portion; first outlet provided on the pressure tank; second outlet provided on the pressure tank; reforming portion configured to use dimethyl ether and water to obtain a reformed gas containing hydrogen with carbon monoxide (CO); CO removing portion that is configured to remove at least a part of the CO contained in the reformed gas; and fuel cell that is configured to use hydrogen contained in the reformed gas and oxygen in the atmosphere to generate electric power.    DESCRIPTION OF DRAWING(S) - The figure shows the construction of a fuel tank for fuel cells.    Dimethyl ether (13)    Water (14)    First outlet (15a)    Second outlet (16a)    Fuel tank (101)    Pressure tank (102)    Dividing portion (103)