To provide a method for cleaning a mask based on contamination of the mask by easily and surely detecting the contamination.

This method comprises a step S1 for deciding a critical size by experiment or simulation for foreign-matter/defect transfer characteristics; a step S2 for setting a small segmented area other than an exposing region on a mask surface, and then dividing the small segmented area into m-pieces of monitoring micro regions; a step S3 for making the mask stay in a column of an exposing device for a specified time period t, and then checking presence of a foreign matter on the m-pieces of monitoring micro regions, to find the number x of the monitoring micro regions in which the foreign matter of a critical size or more exists; a step S4 for finding probability p of occurrence of defect of the mask based on the number x of the monitoring micro region in which the foreign matter exists; a step S5 for finding Tc of the probability p being a threshold pc that makes production yield a specified value or below, to set a cleaning period T with Tc as T; and a step S6 for cleaning the mask at the cleaning period T.