Calibration of the light emitting elements in an optical printer is carried out by mounting the optical printer on an external monitoring apparatus. The external monitoring apparatus comprises: a sensor mounting portion whereon a plurality of photosensors are mounted in a straight line; a signal processing circuit for processing the signals output from the photosensors; a reference voltage generating circuit; and a comparison and control circuit for comparing the output from the signal processing circuit with the reference voltage from the reference voltage generating circuit and outputting a signal based on the results of the comparison. The photosensors receive the light output from the light emitting elements, being subject to calibration, in the optical printer. The signal processing circuit comprises a plurality of integrating circuits which receive the signals output from the plurality of photosensors and integrate those signals over a prescribed period of time; and an arithmetic circuit for calculating the mean value of the output of the integrating circuits and outputting the calibration voltage.