The authors evaluated the performance of an automatic counting for CN film etch-pits in large area as 1cmcm2 by an image processing method. Since the gray-level of the etch-pit image made from a fixed α-particle energy (_??_ 4MeV) is almost constant after shading correction processing, it is expected to get the same counts as manual counts by determining suitable threshold at binarization. The decrease of counts due to pile-up of the etch-pit image could be corrected by calculating the pile-up probability geometrically. Using the image processing method and the pile-up correction, they could get the same counts by automatic counting and manual counting methods for the etch-pits ranging 0-4, 000tracks/cmcm2 made from 2MeV α-particles and 0-1, 000tracks/cmcm2 obtained with cup radon monitors in various living environments.