An endoscope apparatus includes an illumination unit, an optical system including an optical lens and a diaphragm unit configured to adjust a size of an opening, an imaging unit including a solid state imaging device and configured to output an image signal, a region setting unit configured to set a region of interest in an image formed by the image signal, an estimation value calculation unit configured to calculate and output an estimation value showing a focus level in the region of interest, an estimation value storage unit configured to store the estimation value, an estimation value comparison unit configured to compare a reference value and the estimation value and outputs the comparison result, and a diaphragm control unit configured to output a control signal for controlling the opening of the diaphragm unit based on the comparison result.
