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 of 20 to 25000 cP; aliphatic unsaturated group-containing polyorganosiloxanes including an aliphatic unsaturated group-containing linear polyorganosiloxane represented by Formula: (in the formula: each R 1 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 becomes 10,000 to 1,000,000 cP), and 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 (the proportion of the number of the aliphatic unsaturated groups in to the total number of the aliphatic unsaturated groups in is more than 50% and not more than 95%); a photoreaction initiator; a silane compound; and an MQ resin having a weight average molecular weight of 2,000 to 2,500,000 and/or an MDQ resin having a weight average molecular weight of 2,000 to 1,000,000; wherein the ratio of the number of the mercaptoalkyl groups in to the number of the aliphatic unsaturated groups in is 0.5 to 3.0; the amount of is 30 to 200 parts by weight per 100 parts by weight of; and the composition has a viscosity at 23°C of 20,000 to 150,000 cP, wherein the viscosity is a value measured at 23°C on a B-type rotational viscometer (Vismetron VDA-L) (manufactured by Shibaura System) with rotors Nos. 2 to 4 at 30 to 60 rpm, and the viscosity at 23°C was measured with a prescribed rotational speed using the rotor No. 2 in the range of 400 cP or less, the rotor No. 3 in the range of above 400 to 1500 cP, and the rotor No. 4 in the range of above 1500 cP. and the weight average molecular weight is a value measured by gel permeation chromatography using a polystyrene calibration curve. a polyorganosiloxane having a mercaptoalkyl group bonded to a silicon atom, the polyorganosiloxane having a viscosity at 23°C of 20 to 25000 cP; aliphatic unsaturated group-containing polyorganosiloxanes including an aliphatic unsaturated group-containing linear polyorganosiloxane represented by Formula: (in the formula: each R 1 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 becomes 10,000 to 1,000,000 cP), and 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 (the proportion of the number of the aliphatic unsaturated groups in to the total number of the aliphatic unsaturated groups in is more than 50% and not more than 95%); each R 1 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 becomes 10,000 to 1,000,000 cP), and 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 (the proportion of the number of the aliphatic unsaturated groups in to the total number of the aliphatic unsaturated groups in is more than 50% and not more than 95%); a photoreaction initiator; a silane compound; and an MQ resin having a weight average molecular weight of 2,000 to 2,500,000 and/or an MDQ resin having a weight average molecular weight of 2,000 to 1,000,000; wherein the ratio of the number of the mercaptoalkyl groups in to the number of the aliphatic unsaturated groups in is 0.5 to 3.0; the amount of is 30 to 200 parts by weight per 100 parts by weight of; and the composition has a viscosity at 23°C of 20,000 to 150,000 cP,