A mark detection method includes a first step for acquiring a signal that is outputted from a detector configured to output a signal of a first level when a detection value is equal to or more than a first threshold value and output a signal of a second level different from the first level when the detection value is less than the first threshold value, while a state of a web is being detected using the detector, a second step for acquiring conveyance information indicative of a state of conveyance of the web, and a third step for determining whether or not to count the signal of the second level outputted by the detector based on the conveyance information.