To provide a membrane reactor system in which the purge of the membrane reactor used for fuel reforming is carried out without providing a nitrogen storage means.

In a water evaporator 4, steam as a raw material fuel is produced and is supplied to a reforming catalytic layer 11 and a pure hydrogen layer 10 through a communicating valve 27 after the temperature of the steam is controlled. Compressed air from a compressor 23 is supplied to the reforming catalyst layer 11 and the pure hydrogen layer 10 through an air purge valve 25 and a supply valve 26. A controller 37 works so as to supply steam to the reforming catalyst layer 11 and the pure hydrogen layer 10 to prevent the temperature near a hydrogen separation membrane 12 from lowering to equal to or below a prescribed temperature to purge all of hydrogen, to supply steam having a temperature below the catalyst activating temperature of the reforming catalyst layer 11 and to stop the supply of the steam when the temperature of the reforming catalyst layer 11 becomes below the catalyst activating temperature, to supply air to purge moisture and then to stop the supply of the air.