We found that a pi-conjugated polymer, poly(arylene ethynylene), shows extremely high magneto-optic activity with a Verdet constant of 2.5 x 10(5) deg.T(-1).m(-1) in Faraday rotation measurements using a static magnetic field. We also conducted the measurements in regioregular polythiophene spincoated films and reproduced the Verdet constant recently reported using a modulation technique, certifying unusually high magneto-optic activity in polymer/organic materials. This fact suggests that the intermolecular interaction of pi-electron plays important role in this phenomenon. (C) 2009 The Japan Society of Applied Physics