To obtain a composition which can be cured into a silicone elastomer reduced in swell and exudation by mixing an alkenyl-containing polydiorganosiloxane with an organohydrogensiloxane, a catalyst and a resinous organosiloxane copolymer.

This composition is an organosiloxane composition curable into a silicone elastomer, comprising 100 pts.wt. alkenyl-containing polydiorganosiloxane (A) having at least two Si-bonded alkenyl groups on the average per molecule, 0.5-100 pts.wt. organohydrogensiloxane (B) having at least two Si-bonded hydrogen atoms on the average per molecule, 5-100ppm, based on the total of components A and B, of a platinum group catalyst (C) and 0.5-10 pts.wt. resinous organosiloxane copolymer (D) having at least one fluorine-containing group on the average per molecule, having at least two alkenyl groups on the average per molecule and having a number-average molecular weight of 400-15,000g/mol.