[en] We developed an automatic device to measure etch-pit density on cellulose nitrate film, which consisted of microscope, monochrome video camera, image processing device, monochrome display and personal computer. We optimized magnification of microscope and threshold level which was used when image data was converted into binary data. Counting characteristics varied with etch-pit density and incident alpha ray energy and angle were examined. As the result, it was made clear that detection limit diameter of etch-pit was about 5 μm and correction for loss of etch-pit density due to overlapping etch-pit image was not necessary up to about 8000 tracks/cm² in case of about 10 μm diameter etch-pits. High reproducibility of automatic counting was confirmed, and we applied the present device to practical measurement with an electrostatic integrated radon monitor. As the results, it was made clear that this device can be applied to long term measurement and practical use. (author)