PURPOSE: To prevent light emitting elements from being driven always at a high level independently of the existence of external light to extend the life of respective elements by changing the driving level of the light emitting element and a reference deciding level deciding the existence of paper in accordance with the level change of external light.

CONSTITUTION: An optical detection part detecting paper in a printer consists of e.g. a light emitting element 2aa and a photodetective element 2ab and the pulse driving level of the light emitting element 2aa is set up in accordance with the photodetective level of variable external light extracted during the drive stopping period of the light emitting element 2aa to be driven by pulses. A decision reference level detecting an objective substance is provided between the photdetective level obtained by the driving of the light emitting element 2aa and the photodetective level of the external light. At the absence of external light, the light emitting element 2aa is driven and the reflected light from paper receiving the irradiated light is detected by the photodetective element 2ab and the detected signal is outputted to a comparator 39.