This organopolysiloxane composition, when cured at room temperature by moisture in the atmosphere, provides a silicone rubber cured product having good self-adhesiveness to a magnesium alloy. The organopolysiloxane composition contains (A) an organopolysiloxane having a hydroxy group and/or a hydrolysable silyl group at both ends of the molecular chain, (B) an organic silicon compound other than (A) and (C), having at least three hydrolysable groups bonded to a silicon atom per molecule, and/or a partial hydrolysis-condensation product thereof, and (C) a silane coupling agent having a specific molecular structure having a carboxylic acid silyl ester bond. Furthermore, a novel compound, having an alkoxysilyl group and a carboxylic acid silyl ester group per molecule, can have improved adhesiveness/bonding properties with respect to a base material due to the effect of carboxylic acid after hydrolysis thereof.