An image processing program for cell observation in which living cells can be distinguished from foreign matter other than living cells during cell observation under wide observation conditions. The image processing program includes a step for capturing first and second images of an object positioned in the observation field of view at a predetermined time interval, a step for extracting a representation of the object shown in the obtained images, steps for aligning the rotational angular orientation of the object in the image plane and computing a correlation value, a step for determining whether the object is a living cell on the basis of the computed correlation value, and steps for outputting the result of the decision made with regard to the object.