To enable a threshold to be easily changed by users when a printer lapses into an off-line state due to the nonconformity of the threshold accompanying the secular variation of a sensor, etc.

In the printer 10, when the threshold for binarizing a detected signal becomes nonconforming and the off-line state arises due to the level fall of a detected signal accompanying the secular variation of a reflection-type photointerrupter for detecting a black mark formed on a rolled sheet, a real-time threshold setting command is sent to the printer 10 from a host computer 20, and the mode is shifted to a threshold setting one (step S103). In the threshold setting mode, the host computer 20 judges whether the current threshold of the printer 10 is abnormal (step S107). If abnormal, a new threshold is formed (step S109), and it is transmitted to the printer 10 (step S110). The new threshold is stored in a flash ROM by the printer 10 (step S111).