1. A computer readable program recording medium storing, for a computer having a data reading section which reads data of an image and 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 selecting a specified image from among an object image group which includes the plurality of still images and in which respective images are continued in a time base direction; a third step selecting a first image to be added from images before the specified image in the time base direction among the object image group 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 fourth step recording data of the addition rate of the first image to be added in association with data of the specified image.