1. An image thickening processing method, wherein a thickening processing is carried out on a binary image made up of a plurality of black picture elements and white picture elements, which are adjacent to one another along vertical and horizontal directions, the method comprising the steps of: i) setting each of the black picture elements as a black picture element of interest, ii) detecting a vertically adjacent picture element, which is adjacent to said black picture element of interest along a single vertical direction, a horizontally adjacent picture element, which is adjacent to said black picture element of interest along a single horizontal direction, and an obliquely adjacent picture element, which is simultaneously adjacent to said vertically adjacent picture element, said horizontally adjacent picture element, and said black picture element of interest, and iii) carrying out the thickening processing by: a) setting said vertically adjacent picture element as a black picture element only in cases where a picture element, which is adjacent to said vertically adjacent picture element even further along said single vertical direction, has been judged as being a white picture element, b) setting said horizontally adjacent picture element as a black picture element only in cases where a picture element, which is adjacent to said horizontally adjacent picture element even further along said single horizontal direction, has been judged as being a white picture element, and c) setting said obliquely adjacent picture element as a black picture element only in cases where all of a picture element, which is adjacent to said obliquely adjacent picture element even further along said single vertical direction, a picture element, which is adjacent to said obliquely adjacent picture element even further along said single horizontal direction, and a picture element, which is adjacent to said obliquely adjacent picture element obliquely on the side opposite to said black picture element of interest, have been judged as being white picture elements.