Misalignment correction pattern formation method and misalignment correction method

A misalignment correction method for forming a plurality of color images on a transfer belt to form a multi-color image and correcting a misalignment of each color using a detection sensor, the method comprising:

• preparing a reference pattern with a plurality of lines formed by a black color as a reference color, the reference pattern having a predetermined pitch between the lines;
• preparing a color pattern to be corrected with a plurality of lines formed by a color toner, the color pattern having an equal pitch to the pitch in the reference pattern;
• forming a patch by superposing the reference pattern as a last color to be superposed on the color pattern;
• preparing a plurality of patches each in which a position of the lines in the color pattern with respect to the lines in the reference pattern is shifted by a shift quantity in a pitch direction of the lines;
• arranging the plurality of patches on the transfer belt along a movement direction thereof;
• selecting as a reference patch, either one of a patch in which the color pattern to be corrected is completely superposed by the reference pattern and a patch in which the color pattern to be corrected is completely displaced from the reference pattern;
• forming a continuously formed patch group including the reference patch and the patches as a correction pattern;
• optically detecting a value of each of the patches with respect to the shift quantity of the respective patches of the continuously formed patch group in the correction pattern;
• calculating an intersection point of two approximate lines as output obtained on opposite sides of an inflection point of the two lines, by using the detected values excluding a minimum value and a maximum value;
• and
• performing misalignment correction based on the intersection point.