A curable composition according to the present invention for sealing optical semiconductor includes components (A), (B), and (C). Another curable composition according to the present invention for sealing optical semiconductor further includes a component (D) in addition to the components (A), (B) and (C). The component (A) is a compound containing at least one functional group selected from the group consisting of epoxy groups, oxetanyl groups, vinyl ether groups, and (meth)acryloyl groups. The component (B) is a cycloaliphatic epoxy compound. The component (C) is a curing catalyst including a cationic component and an anionic component and generating an acid upon application of light or heat, where the cationic component contains an aromatic ring, and the anionic component contains a central element selected from boron and phosphorus. The component (D) is conductive fiber-bearing particles each including a particulate substance and a fibrous conductive substance lying on or over the particulate substance.