To prevent a malfunction during the check of an information processor by surely setting an optimum threshold without much labor, depending on an individual difference among the sensors provided in the information processor.

A printer 10 as a test object is started up in the state of being connected to a threshold setting device 20. In that case, it is determined whether or not an initial threshold is set in the predetermined area of a flash ROM 102 which is provided in the printer 10, and the printer 10 is put into a threshold setting mode when the initial threshold is not set in the predetermined area of the flash ROM 102. In the threshold setting mode, a black mark of reference paper is detected so that the threshold can be computed and stored in the predetermined area of the flash ROM 102.