To obtain the subject new compound having one or more episulfide structure each containing three-membered ring structure-constituting sulfur atoms in a specific value or larger, capable of simultaneously imparting high heat resistance and high strength to optical material having good balances between reflective indexes and Abbe numbers, and useful as a raw material for spectacle plastic lenses.

A compound of formula II (X is S or O wherein the number of S atoms is on the average ≥50% based on the total number of the S and O atoms in X; R1-R3 are each H or a 1-10C hydrocarbon group; R4 is a 0-10C hydrocarbon chain) excluding a compound of formula I [Eps is a (thio)glycidyl group wherein the number of S atoms is on the average ≥50% based on the total number of the S and O atoms in the group; (l) is 1-6; (m) is 0-4], etc. For example, 2,6-dimethyl-1,2:6,7-diepithio-4-thiaheptane. The compound of formula II is obtained by reacting a mercapto-containing compound with a (alkyl) epihalohydrin in the presence of an alkali and subsequently reacting the obtained compound of formula II (X is O) with a thio group-introducing agent such as a thiocyanate salt.