A halftone phase shift film is formed on a transparent substrate by reactive sputtering using a silicon target, an inert gas, and a nitrogen-containing reactive gas. A hysteresis curve is drawn by sweeping the flow rate of the reactive gas, and plotting the sputtering voltage or current during the sweep versus the flow rate of the reactive gas. In a transition mode sputtering step of sputtering in a region corresponding to a range from more than the lower limit of reactive gas flow rate providing the hysteresis to less than the upper limit, the target power, the inert gas flow rate and/or the reactive gas flow rate is increased or decreased continuously or stepwise.