A pressure sensitive adhesive layer-forming organopolysiloxane composition, comprising: (A) a chain organopolysiloxane having alkenyl groups in numbers greater than 1 on average within the molecule; (B) an organopolysiloxane resin mixture comprising the following components (b1) and (b2) at a mass ratio of 1:99 to 99:1: (b1) an organopolysiloxane resin, wherein the total content of hydroxyl groups and hydrolyzable groups with respect to all silicon atoms in the molecule is 9 mole% or less, and the weight average molecular weight (Mw) measured in terms of standard polystyrene by gel permeation chromatography (GPC) is 4500 or more; and (b2) an organopolysiloxane resin, wherein the total content of hydroxyl groups and hydrolyzable groups with respect to all silicon atoms in the molecule is 9 mole% or less, and the weight average molecular weight (Mw) measured in terms of standard polystyrene by gel permeation chromatography (GPC) is less than 4500; (C) an organohydrogenpolysiloxane having at least two Si-H bonds in the molecule; and (D) an effective amount of a hydrosilylation reaction catalyst, wherein the mass ratio of component (B) to component (A) is within a range of 0.9 to 2.0, and the shear storage elastic modulus G' determined in accordance with the method outlined in the description at -20°C of a pressure sensitive adhesive layer obtained by curing the composition is within a range of 0.01 to 0.75 MPa. (A) a chain organopolysiloxane having alkenyl groups in numbers greater than 1 on average within the molecule; (B) an organopolysiloxane resin mixture comprising the following components (b1) and (b2) at a mass ratio of 1:99 to 99:1: (b1) an organopolysiloxane resin, wherein the total content of hydroxyl groups and hydrolyzable groups with respect to all silicon atoms in the molecule is 9 mole% or less, and the weight average molecular weight (Mw) measured in terms of standard polystyrene by gel permeation chromatography (GPC) is 4500 or more; and (b2) an organopolysiloxane resin, wherein the total content of hydroxyl groups and hydrolyzable groups with respect to all silicon atoms in the molecule is 9 mole% or less, and the weight average molecular weight (Mw) measured in terms of standard polystyrene by gel permeation chromatography (GPC) is less than 4500; (b1) an organopolysiloxane resin, wherein the total content of hydroxyl groups and hydrolyzable groups with respect to all silicon atoms in the molecule is 9 mole% or less, and the weight average molecular weight (Mw) measured in terms of standard polystyrene by gel permeation chromatography (GPC) is 4500 or more; and (b2) an organopolysiloxane resin, wherein the total content of hydroxyl groups and hydrolyzable groups with respect to all silicon atoms in the molecule is 9 mole% or less, and the weight average molecular weight (Mw) measured in terms of standard polystyrene by gel permeation chromatography (GPC) is less than 4500; (C) an organohydrogenpolysiloxane having at least two Si-H bonds in the molecule; and (D) an effective amount of a hydrosilylation reaction catalyst, wherein the mass ratio of component (B) to component (A) is within a range of 0.9 to 2.0, and the shear storage elastic modulus G' determined in accordance with the method outlined in the description at -20°C of a pressure sensitive adhesive layer obtained by curing the composition is within a range of 0.01 to 0.75 MPa. <EOS>