PURPOSE: To engrave a Fresnel step at a low cost, by a method wherein a bite having an acute tip attached to a milling machine is reciprocatively cut at each width of each Fresnel step and is radially scanned and moved at a fine pitch to cut out a given crest angle and a slope.

CONSTITUTION: In each arcuate Fresnel step plan section 2, inclination angles α1 and α2 of both ends of an inner slope 4 and inclination angles β1, and β2 of both ends of an outer slope 5 are planned so that beams l1 and l2 from a focus F on an optical axis L of a lens member 1 forms beams paralleling the optical axis L to decide apexes γ1 and γ2 of each Fresnel step 3. When the plan section 2 is out by a multishaft milling machine, a bite 10 having an acute-angled tip is reciprocatively moved in the direction of the length of the Fresnel step 3 for cutting, and is radially moved in a scanning manner at a fine pitch to cut out a crest angle and a slope. In this case, change amounts α1 → α2 and β1 → β2 ot the inclination angle of the Fresnel step 3 are produced as a backward movement amount of the bite 10 is gradually varied.