To suppress resistance to mould release caused by resin contraction produced by cooling solidification during injection moulding, when performing injection moulding of a plastic lens.

Even in a comparatively deep range in which the value (Y/X) of the horizontal/vertical ratio of the step shapes of, for example, a three-wavelength-compatible optical element satisfies 0.50≤ maximum value of (Y/X) ≤1.0, by performing mold release in which a first optical transfer face 11A having step shapes 13A, 15A that form a diffraction structure is released from a plastic lens PL before a second optical transfer face 21A is released, the plastic lens PL can be smoothly released from a fixed mold 10 with substantially no resistance and even in the case of an optical element having deep steps in a microstructure, thereby making it possible to prevent deformation of the steps, namely the micro shapes, of the diffraction structure of the plastic lens PL.