The measurement of the size of moving holes is described by processing their image using spatial filtering and converting their size to electrical signal. In the present case, the shape of the hole was assumed to be circular and the band-pass filter giving high sensitiv-ity was designed. In deciding the aperture size for the photodetector, tolerance to position fluctuation of the moving holes was taken into consideration. Dealing with the holes of various size within 30011m ±10% diameter, the accuracy obtained was within l%,i.e. 3μm. Moving speed of the holes was 20cm/sec and their position fluctuation was allowable near to their size.