A digital still camera of the present invention has a photographing optical system, a body, an image sensor, a half mirror, a phase difference focus detector. The image sensor is arranged in the body and backward of the photographing optical system along an optical axis. The light-path space is formed between the photographing optical system and the image sensor along the optical axis, light from the photographing optical system passes through the light-path space and is received by the image sensor. The half mirror is provided along the optical axis in the light-path space and the phase difference focus detector is provided outside the light-path space. The half mirror is arranged in the light-path space such that all of the light directed from the photographing optical system to the image sensor substantially passes through the half mirror.