The present invention provides a process for the preparation of a reactive silicon group-containing polymer. A novel process for the preparation of the polymer includes allowing an alkylene oxide to undergo ring opening polymerization with an organopolysiloxane compound having an active hydrogen-containing functional group with which the alkylene oxide is reactive as an initiator in the presence of a catalyst to synthesize a polymer of organopolysiloxane and polyoxyalkylene, and then introducing a reactive silicon group into the polymer. Another process for the preparation of the polymer as defined above is provided, which includes allowing an alkylene oxide to undergo ring opening polymerization with an organopolysiloxane compound having an epoxy group in the presence of a catalyst with an active hydrogen-containing compound as an initiator to synthesize a polymer the main chain of which is essentially a copolymer of organopolysiloxane and polyoxyalkylene, and then introducing a reactive silicon group into the polymer.