Patent ID: 8526873

Claim:
An image forming apparatus that forms an image on a web with reference to a position of a positioning mark on the web, the image forming apparatus comprising: a moving unit that causes a relative movement of the web and a mark detecting unit in a predetermined direction, the mark detecting unit detecting a ground color of the web and a mark color which is a color of the positioning mark; and a control device that measures a measurement value corresponding to a moving amount of the mark detecting unit with respect to the web in a forward direction in a time from when a color detected by the mark detecting unit is switched from the ground color to the mark color till when a color detected by the mark detecting unit is switched back to the ground color, wherein the control device causes the moving unit to relatively move the web and the mark detecting unit in a reverse direction for a distance of half the measurement value from when the color detected is switched back to the ground color and measures a mark detection signal that is detected by the mark detecting unit at this distance, and wherein the control device sets a threshold used for determining whether the color detected is the ground color or the mark color on the basis of the measured mark detection signal.