The present invention relates to a technology of appropriately improving image resolution. According to an image signal processing method of the present invention, an input video signal contains an image signal whose pixel size is converted at a specified magnification scale. The input image signal is used to calculate sampling frequencies for image signals before and after the pixel size conversion. A ratio of sampling frequencies before and after the pixel size conversion is used to determine the specified magnification scale. A sampling phase for the image signal before the pixel size conversion is calculated from the input image signal after the pixel size conversion. A sampling carrier for the image signal before the pixel size conversion is generated from the sampling frequency and the sampling phase both before the pixel size conversion.