1. A binarizing method for converting an image picked up by a television camera into a binary image based on a binarization level used as a threshold, comprising: a first step of finding a maximum value and a minimum value of a density of the image picked up; a second step of creating a histogram of the density of the image and dividing the histogram into a first region and a second region based on the maximum and minimum values of the density of the image and a predetermined first division ratio; a third step of calculating an average density of each of the first region and the second region; a fourth step of deciding a binarization level based on the average density of each of the first and second regions and a second division ratio; a fifth step of binarizing the image based on the binarization level obtained in the fourth step; a sixth step of detecting a pattern by means of a binarized image obtained in the fifth step; and a seventh step of altering the binarization level by a predetermined degree and performing the fifth and sixth steps if the pattern is not correctly detected at the sixth step and storing the binarization level in order to set the second division ratio in the fourth step based on the binarization level if the pattern is correctly detected at the sixth step.