A compact, energy efficient image sensor which enables transfer of images to an external system is made compact in size and low in power consumption comprises an image sensor control circuit 20 which operates to drive a CCD image sensor 4 and stores horizontal lines of image data in a line memory 30. An HREF signal is raised in association with the storing operation. The external system monitors the HREF and supplies an external clock EXCLK to an image sensor 2 during an H level period according to its convenience. A memory control circuit 34 sequentially outputs image data from the line memory 30 to a data bus 10 in pixel units and in synchronization with the EXCLK.