1. An image binarization method, wherein as a first processing, multi-digitized luminance data obtained by digitally converted video signals from an imaging device for each pixel on each horizontal scanning line is stored in an even-numbered row horizontal line memory if the current horizontal scanning line is an even-numbered row, and is stored in an odd-numbered row horizontal line memory if the horizontal scanning line is an odd-numbered row, and in said multi-digitized luminance data on the current scanning line, detected maximum value MAX i and minimum value MIN j exceeding a predetermined displacement level, and addresses MAXP i and MINP j at the detected pixel positions are stored in an even-numbered detection memory if the current horizontal scanning line is an even-numbered row, or in an odd-numbered row detection memory if the current horizontal scanning line is an odd-numbered row; and as a second processing, reading-out is carried out from the even-numbered detection memory if the current horizontal scanning line is an odd-numbered row, or from the odd-numbered detection memory if the current horizontal scanning line is an even-numbered row, and based on floating thresholds FT h &equals;MIN j &plus;(MAX i −MIN j )×K (herein, K is an emphasis coefficient between 0 and 1, and h, i, and j are integers starting from zero) for each section X h of the horizontal pixel address row set by means of operation, the multi-digitized data read out from the even-numbered horizontal line memory in the case of the odd-numbered row or the odd-numbered horizontal line memory in the case of the even-numbered row is converted into binary data.