PURPOSE: To prevent erroneous diagnosis by photographing a picture having a known density and storing it in a picture memory and scanning this stored data successively to regard data positions other than the known density as error positions.

CONSTITUTION: The output signal of a solid-state image pickup element at the front end of an endoscope probe is temporarily stored in a memory, and the picture having a know density photographed by this element and stored is scanned. The position of data other than the known density is detected, and correction is performed on a basis of this error data or the position is displayed. Thus, error due to break of the solid-state image pickup element, dust, or the like is detected without replacing the front end of the probe to prevent erroneous diagnosis based on error.