PURPOSE: To provide the subject composition containing a thermosetting resin and a specific copolymer at a specific ratio, having a prescribed dispersed particle diameter, exhibiting excellent processability, impact resistance, strength and electrical properties and useful as a semiconductor sealing material, etc.

CONSTITUTION: The objective composition contains (A) 100 pts.wt. of a thermosetting resin such as phenolic resin and (B) 1-100 pts.wt. of a copolymer obtained by the radical polymerization of a crosslinkable polymer having ≥2 unsaturated bonds in one molecule (e.g. divinylbenzene) and other radically polymerizable organic monomer (e.g. methyl acrylate) in the presence of particulate polyorganosiloxane having the structural unit of formula [R is (substituted) univalent organic group containing 0.02-10mol% of ethylenically unsaturated group; (a) is 1.80-2.02]. The gel content of the copolymer is ≥50wt.% and the particle diameter of the component B dispersed in the thermoplastic resin is 0.1-1μm.