PURPOSE: To mass-produce economically lenses having a uniform focal length, by bringing at least one surface of a plastic flat plate that is in a semipolymerized state into contact with the surface of a belt-like masking material which is provided with a plurality of arranged circular projections or through-holes, and passing them through a monomer solution within a certain dwell time.

CONSTITUTION: The belt-like masking material 2 and the plastic flat plate 1 that is in a semi-polymerized state are passed through a monomer solution 6 with the tops of the circular projections 3 arranged on the masking material 2 being in contact with the plastic flat plate 1. Said circular projections 3 serve as masks, and the monomer present between the circular projections 3 is polymerized with the plastic flat plate 1 that is in a semi-polymerized state, so that the lenses 8 corresponding to the circular projections 3 on the belt-like masking material 2 are formed on the plastic flat plate 1. Thus, the lenses having a uniform focal length can be mass-produced continuously and inexpensively.