An imaging system for use in an endoscope includes a vari-focal lens system provided in a viewing head to be inserted into a cavity or opening of a living body or machinery, a solid-state image sensor for imaging an image formed by the lens system to produce electrical signals representative of the image, a supporting unit for supporting the lens system and the image sensor thereon so as to change the spacing between the lens system and the image sensor on the optical axis of the lens system, and controller circuitry operable in response to magnification of the lens system for estimating the position of a focus of the lens system to control the supporting unit so as to position the image sensor substantially at the estimated position of the focus.