A digital camera includes a CCD imager. If brightness of an object is not enough, a frame rate of the CCD imager at a time of a focus adjustment is changed from 30 fps to 15 fps. This also lowers a reading speed of a camera signal, and reduces noise included in the camera signal. An AF evaluation value is acquired by integrating a high frequency component of a Y signal generated on the basis of the camera signal, and therefore, high noise causes an error in the AF evaluation value and fails to correctly adjust focus. This is the reason why the driving speed of the CCD imager is lowered when the brightness of the object is not enough.