Provided is a method of producing a high molecular weight organopolysiloxane with a polystyrene equivalent weight average molecular weight of at least 5.times.10.sup.4, comprising the steps of producing an organopolysiloxane by subjecting a silane compound having a hydrolyzable group to a first hydrolysis and condensation, and then subjecting that organopolysiloxane to an additional second hydrolysis and condensation. The high molecular weight organopolysiloxane is stable, resistant to gelling, and resistant to cracking even when formed as a thick film. A resin composition comprising the high molecular weight organopolysiloxane and a condensation catalyst is useful for sealing an optical element and for producing an optical semiconductor device.