A process for producing a plastic lens which comprises preparing a prepolymer comprising sulfur by bringing at least one compound having epithio group, sulfur and a portion of a polythiol compound into reaction with each other and, thereafter, adding rest of the polythiol compound to the prepared prepolymer comprising sulfur and allowing polymerization to proceed. A plastic lens of excellent quality which exhibits excellent weatherability in combination with a great refractive index, a great Abbe number and excellent transparency is produced.