An organopolysiloxane composition with pressure sensitive adhesive layer forming properties, comprising: (A) an organopolysiloxane having an average of more than one alkenyl group in a molecule, comprising a mixture of (A1) a raw rubber-like organopolysiloxane containing an alkenyl group, having a viscosity at 25°C of 100,000 mPa·s or more, or a degree of plasticity as measured in accordance with a method stipulated in JIS K6249 within a range of 50 to 200, and (A2) an organopolysiloxane containing an alkenyl group with a viscosity at 25°C that is less than 100,000 mPa·s, and a mass ratio of both components is within a range of 50:50 to 100:0, and an amount of a vinyl (CH 2 = CH) moiety in an alkenyl group in component (A1) is within a range of 0.005 to 0.400 wt.%; (B) an organopolysiloxane resin wherein at least a portion of component (B) is (B1) a resin that contains an R 3 SiO 1/2 unit (M unit) and an SiO 4/2 unit (Q unit), and may have a hydroxyl group or hydrolyzable group (where R represents a monovalent organic group and 90 mol% or more of R is a phenyl group or alkyl group with 1 to 6 carbon atoms); and component (B) is within a range of 1 to 500 parts by mass with regard to 100 parts by mass of a sum of component (A) and component (C) in the composition; (C) an organohydrogen polysiloxane having at least two Si-H bonds in a molecule; (D) a hydrosilylation reaction catalyst; and optionally (E) at least one type of curing retarder; wherein an amount of the component (C) is an amount such that a ratio (molar ratio) of an amount (moles) of SiH groups in component (C) with regard to a sum of an amount (moles) of alkenyl groups in component (A) and amount (moles) of alkenyl groups in component (B) is 20 to 60; and each viscosity is measured using a rotational viscometer in accordance with JIS K7117-1. (A) an organopolysiloxane having an average of more than one alkenyl group in a molecule, comprising a mixture of (A1) a raw rubber-like organopolysiloxane containing an alkenyl group, having a viscosity at 25°C of 100,000 mPa·s or more, or a degree of plasticity as measured in accordance with a method stipulated in JIS K6249 within a range of 50 to 200, and (A2) an organopolysiloxane containing an alkenyl group with a viscosity at 25°C that is less than 100,000 mPa·s, and a mass ratio of both components is within a range of 50:50 to 100:0, and an amount of a vinyl (CH 2 = CH) moiety in an alkenyl group in component (A1) is within a range of 0.005 to 0.400 wt.%; (B) an organopolysiloxane resin wherein at least a portion of component (B) is (B1) a resin that contains an R 3 SiO 1/2 unit (M unit) and an SiO 4/2 unit (Q unit), and may have a hydroxyl group or hydrolyzable group (where R represents a monovalent organic group and 90 mol% or more of R is a phenyl group or alkyl group with 1 to 6 carbon atoms); and component (B) is within a range of 1 to 500 parts by mass with regard to 100 parts by mass of a sum of component (A) and component (C) in the composition; (C) an organohydrogen polysiloxane having at least two Si-H bonds in a molecule; (D) a hydrosilylation reaction catalyst; and optionally (E) at least one type of curing retarder; wherein an amount of the component (C) is an amount such that a ratio (molar ratio) of an amount (moles) of SiH groups in component (C) with regard to a sum of an amount (moles) of alkenyl groups in component (A) and amount (moles) of alkenyl groups in component (B) is 20 to 60; and each viscosity is measured using a rotational viscometer in accordance with JIS K7117-1. <EOS>