<p>Disclosed is an isobutylene block copolymer which is hardly colored and has a stable color tone. Specifically disclosed is an isobutylene polymer obtained by polymerizing a monomer component containing isobutylene in the presence of a polymerization initiator and a Lewis acid catalyst. This isobutylene polymer is characterized in that the residual amount of metal derived from the Lewis acid catalyst in the isobutylene polymer is not more than 90 ppm. Also disclosed is a method for producing such an isobutylene polymer.</p>
