According to a program, a computer determine an interpolation method for interpolating respective pixel values of plural pixels not existing in an input image and positioned on a straight line. Specifically, the computer selects a target pixel from among the plural pixels sequentially; calculates a local correlation value for each correlation calculation direction on the basis of pixel values of first and second pixels respectively existing in the correlation calculation direction and a direction opposite thereto viewed from the target pixel; calculates a cumulative correlation value for each correlation calculation direction on the basis of both the local correlation value and the cumulative correlation value related to a prior pixel selected immediately prior; selects one correlation calculation direction on the basis of the cumulative correlation values; and determines plural pixels for interpolation on the basis of the local and/or the cumulative correlation value (s) corresponding to the selected correlation calculation direction.