An image display sealant dam composition that comprises: a polyorganosiloxane having a mercaptoalkyl group bonded to a silicon atom, the polyorganosiloxane having a viscosity at 23°C measured on a B-type rotational viscosimeter at 30 to 60 rpm of 20 to 25000 mPa·s; an aliphatic unsaturated group-containing polyorganosiloxane including an aliphatic unsaturated group-containing linear polyorganosiloxane represented by Formula: (in the formula: each R1 group independently represents an aliphatic unsaturated group, each R group independently represents a C1 to C6 alkyl group or a C6 to C12 aryl group, and 1 to 60 mol% of the total R groups are C6 to C12 aryl groups, and the letter n is a value such that the viscosity at 23°C measured on a B-type rotational viscosimeter at 30 to 60 rpm becomes 100 to 1,000,000 mPa·s), and optionally further including a branched polyorganosiloxane having SiO 4/2 units, R' 3 SiO 1/2 units and R' 2 SiO 2/2 units, and optionally further having R'SiO 3/2 units (in the formulae, each R' group is independently a C1 to C6 alkyl group or an aliphatic unsaturated group), at least three R' groups per molecule of the branched polyorganosiloxane are aliphatic unsaturated groups; a photoreaction initiator; an aliphatic unsaturated group-containing silane compound; and a hexamethyldisilazane-treated fumed silica having a BET specific surface area of 300 to 500 m 2 /g; wherein the ratio of the number of the mercaptoalkyl groups present in to the total number of the aliphatic unsaturated groups in and is 0.45 to 1.50; the amount of is 0.5 to 24 parts by weight per 100 parts by weight of; the composition has a viscosity at 23°C measured on a B-type rotational viscosimeter at 30 to 60 rpm of 20,000 to 10,000,000 mPa·s. a polyorganosiloxane having a mercaptoalkyl group bonded to a silicon atom, the polyorganosiloxane having a viscosity at 23°C measured on a B-type rotational viscosimeter at 30 to 60 rpm of 20 to 25000 mPa·s; an aliphatic unsaturated group-containing polyorganosiloxane including an aliphatic unsaturated group-containing linear polyorganosiloxane represented by Formula: (in the formula: each R1 group independently represents an aliphatic unsaturated group, each R group independently represents a C1 to C6 alkyl group or a C6 to C12 aryl group, and 1 to 60 mol% of the total R groups are C6 to C12 aryl groups, and the letter n is a value such that the viscosity at 23°C measured on a B-type rotational viscosimeter at 30 to 60 rpm becomes 100 to 1,000,000 mPa·s), and optionally further including a branched polyorganosiloxane having SiO 4/2 units, R' 3 SiO 1/2 units and R' 2 SiO 2/2 units, and optionally further having R'SiO 3/2 units (in the formulae, each R' group is independently a C1 to C6 alkyl group or an aliphatic unsaturated group), at least three R' groups per molecule of the branched polyorganosiloxane are aliphatic unsaturated groups; each R1 group independently represents an aliphatic unsaturated group, each R group independently represents a C1 to C6 alkyl group or a C6 to C12 aryl group, and 1 to 60 mol% of the total R groups are C6 to C12 aryl groups, and the letter n is a value such that the viscosity at 23°C measured on a B-type rotational viscosimeter at 30 to 60 rpm becomes 100 to 1,000,000 mPa·s), and optionally further including a branched polyorganosiloxane having SiO 4/2 units, R' 3 SiO 1/2 units and R' 2 SiO 2/2 units, and optionally further having R'SiO 3/2 units (in the formulae, each R' group is independently a C1 to C6 alkyl group or an aliphatic unsaturated group), at least three R' groups per molecule of the branched polyorganosiloxane are aliphatic unsaturated groups; a photoreaction initiator; an aliphatic unsaturated group-containing silane compound; and a hexamethyldisilazane-treated fumed silica having a BET specific surface area of 300 to 500 m 2 /g;