The invention discloses an information recording medium processing device and a signal processing method. When a light emitting diode (LED) used for detecting the transmission position of an information recording medium is directly driven by an external power supply, normal judgment can be executed even though a detecting signal of a photosensitive diode (PD) varies because of the lighting emitting state of the LED variation caused by the variation of the external power supply. For example, when a card reader is produced, the lower detecting value and the upper detecting value of the PD is calculated when lower limit voltage and upper limit voltage are applied to the LED, and a correction coefficient in an area from the lower limit voltage to the upper limit voltage is calculated and stored in a nonvolatile storage in advance. When the measured voltage of the external power supply is in a range from the upper limit voltage to the lower limit voltage, a calculation control unit corrects the PD with the correction coefficient [alpha]. If a corrected value is over a first threshold value, the light from the LED is determined to be received by the PD normally; if the corrected value is under a second threshold value, the light from the LED is determined not to be received by the PD normally.