An organic EL element driving device that drives an organic EL element with a constant current is provided. The hub includes a pair of tapered regions near ends of its inner peripheral surface. The rotor further includes at least one impeller wheel having an impeller wheel hub and a sealing segment arranged between the gear tooth segment and the impeller wheel hub that bears a seal. The reaction mixture contains less than about 3000 ppm, preferably less than about 500 ppm, more preferably less than about 100 ppm of oxidized copper during the reaction.