Super-resolution processing is regarded as a kind of intersample interpolation process and performed by using a linear interpolation filter designed according to sampled-data control theory. In an error-system model used in the process of designing the filter, a pre-filter is disposed before a sampler and a post-filter is disposed after the zero-order hold in a signal restoration system. At least either one of the characteristic Q(s) of the pre-filter or the characteristic P(s) of the post-filter is designed to be a low-pass characteristic, whereby the gain of the characteristic K(z) of a digital filter within a middle-to-high frequency range is raised. Raising the gain makes jaggies more prominent. To reduce this effect, in an actual processing system, the image to be processed is passed through a low-pass filter to reduce high-frequency components before the linear interpolation is performed.