Image information stored on a recording medium is read out by photoelectrically converting light obtained by two-dimensional scanning of the image information, and integrating the electric signal by use of a plurality of integrators which are sequentially actuated each time a first clock pulse determining the picture element size is input. Integration periods are set not shorter than one clock length of the first clock pulse so that they partially overlap each other. The integration output is sampled by a second clock pulse at a point of time within the overlap region of the integration periods but outside regions where the next integrator is in the integration start transient and where the currently operating integrator is reset. The integrator reset period is set to sufficiently reset each integrator and prepare for the next integration.