1. A non-transitory computer readable program recording medium storing, for a computer having a data reading section which reads data of an image and having a calculating section, a program to cause the calculating section to execute: a first step reading, into the calculating section, data of a plurality of still images generated by continuously imaging still images of a subject using an imaging apparatus; a second step generating an interpolation image by interpolating change in position of the subject between two images of the plurality of still images; a third step determining a specified image from among an object image group which includes the plurality of still images and the interpolation image, respective images in the object image group being continued in a time base direction; a fourth step determining a first image to be added from images among the object image group before the specified image in the time base direction and with different positions of the subject in order to generate a composite image by adding the first image to be added before the specified image in the time base direction to the specified image and determining an addition rate of the first image to be added to the specified image; and a fifth step recording data of the first image to be added and the addition rate in correspondence with data of the respective images in the object image group in association with the data of the respective images.