In a phase shift layer forming step, by setting the flow rate ratio of an oxidizing gas in an ambient gas, phase shift layers (11b, 11c, 11d) are formed on a transparent substrate (S) in multiple stages. Further, in a phase shift pattern forming step, the phase shift layers are weight-etched to form a multistage region (B1bh, B1bi) in which the change in thickness in the phase shift layer is configured in multiple stages.