To provide a plate making method for a planographic printing plate using a positive-working IR sensitive planographic printing plate excellent in contrast between image and non-image areas, capable of forming a highly sharp and clear image and coping with a wide range of development conditions.

In the plate making method, the positive-working IR sensitive planographic printing plate obtained by disposing an image forming layer containing an IR absorbent on a support in which pits formed by electrolytic surface roughening in hydrochloric acid have 5-30 μm average pit diameter and ≥60% of the total number of the pits has 5-30 μm pit diameter is exposed with IR and developed with an alkali developing solution containing 5.0-10 mass% (expressed in terms of silicon dioxide (SiO2)) alkali silicate.