The present invention uses a curable resin which has: in the molecule thereof, at least two crosslinking functional groups of at least one kind selected from the group consisting of epoxy group, (meth)acryloyl group, alkenylamino group and alkenyloxy group; a glass transition temperature before curing of 50 to 150.degree. C.; a weight-average molecular weight of 10,000 to 1,000,000; and a dielectric constant of not higher than 3.5 after curing. By use of the curable resin of the above constitution, the present invention can provide a curable resin that an insulator having an excellent thermal shock resistance and an excellent dielectric property is obtained.