An optical image transmitted through a photographing lens is incident on a light-receiving unit of a two-dimensional matrix. An output from the light-receiving unit is input to a first arithmetic logic unit, and the first arithmetic logic unit calculates actual object brightness values in consideration of an aperture value of an aperture. An output from the first arithmetic logic unit is supplied to a multiplexer and a neural network. The neural network determines a main part of the object from a pattern of brightness values of the respective photoelectric transducer elements and outputs a position signal of the main part. The multiplexer selectively passes the brightness value of the photoelectric transducer element corresponding to the main part of the object from the outputs generated by the first arithmetic logic unit. An output from the multiplexer is supplied to a second arithmetic logic unit. The second arithmetic logic unit performs a focus detection calculation based on only the brightness of the main part. The photographing lens is moved along the optical axis, thereby performing a focusing operation.