An organopolysiloxane composition for bonding to magnesium alloys comprises (A) 100 parts by weight of an organopolysiloxane of the following general formula (1) and/or (2) ##STR00001## wherein R may be the same or different and represents a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms, and n is an integer of at least 10, X represents an oxygen atom or an alkylene group having 2 to 5 carbon atoms, and m independently represents an integer of 0 or 1, (B) 0.1 to 50 parts by weight of an organosilicon compound having at least three hydrolyzable groups bonded to silicon atoms in one molecule and/or a partial hydrolyzate thereof, and (C) 0.1 to 15 parts by weight of an acidic silane coupling agent.