The invention aims at a curing agent, curable compositions or compositions for optical materials, capable of providing for cured artifacts having laudable heat resistance, optical transparency, and light resistance; optical materials and methods for their production; and liquid crystal displays and LED's made by using the materials. A curing agent, which comprises at least two SiH groups, is obtainable by subjecting an aliphatic organic compound (.alpha.1) and an acyclic and/or cyclic polyorganosiloxane (.beta.1) to hydrosilylation reaction. The curing agent is also obtainable by subjecting an organic compound (.alpha.2) and a cyclic polyorganosiloxane (.beta.2) to hydrosilylation reaction.