A moving unit causes a relative movement of a web and a mark detecting unit for detecting a ground color of the web and a mark color in a predetermined direction. A measuring unit measures a measurement value corresponding to a moving amount of the mark detecting unit with respect to the web in a time from when a detected color is switched from the ground color to the mark color till when a detected color is back to the ground color. A setting unit sets a threshold used for determining whether the color detected is the ground color or the mark color on the basis of the mark color detected by the mark detecting unit at the time when the mark detecting unit is moved in a reverse direction for a distance of half the measurement value since the color detected is back to the ground color.