A computer program for an image processing device including a storage unit that stores therein image data including time-series computed tomography images in a plurality of frames, of an organ of a subject captured after a contrast medium has been administered, wherein the computer program causes the image processing device to execute a first step of determining a change-over-time of a CT value based on image data including CT images in the plurality of frames, a second step of determining a predetermined slope that is a slope of the CT value with respect to a predetermined time based on a change-over-time of the CT value determined in the first step, and a third step of approximating a change-over-time of the CT value with a predetermined function based on the predetermined slope determined in the second step, characterized in that the image data includes first image data including CT images in a first predetermined number of frames, and second image data including CT images in a second predetermined number of frames that is smaller than the first predetermined number, and the computer program causes the image processing device to further execute a fourth step of determining a maximum timing that is a timing at which the CT value becomes a maximum value in a change-over-time of the CT value determined in the first step based on the first image data, and to execute the first step, the second step, and the third step based on the second image data including CT images in the second predetermined number of frames based on the maximum timing determined in the fourth step. the computer program causes the image processing device to execute a first step of determining a change-over-time of a CT value based on image data including CT images in the plurality of frames, a second step of determining a predetermined slope that is a slope of the CT value with respect to a predetermined time based on a change-over-time of the CT value determined in the first step, and a third step of approximating a change-over-time of the CT value with a predetermined function based on the predetermined slope determined in the second step, characterized in that the image data includes first image data including CT images in a first predetermined number of frames, and second image data including CT images in a second predetermined number of frames that is smaller than the first predetermined number, and the computer program causes the image processing device to further execute a fourth step of determining a maximum timing that is a timing at which the CT value becomes a maximum value in a change-over-time of the CT value determined in the first step based on the first image data, and to execute the first step, the second step, and the third step based on the second image data including CT images in the second predetermined number of frames based on the maximum timing determined in the fourth step. a first step of determining a change-over-time of a CT value based on image data including CT images in the plurality of frames, a second step of determining a predetermined slope that is a slope of the CT value with respect to a predetermined time based on a change-over-time of the CT value determined in the first step, and a third step of approximating a change-over-time of the CT value with a predetermined function based on the predetermined slope determined in the second step, characterized in that the image data includes first image data including CT images in a first predetermined number of frames, and second image data including CT images in a second predetermined number of frames that is smaller than the first predetermined number, and the image data includes first image data including CT images in a first predetermined number of frames, and second image data including CT images in a second predetermined number of frames that is smaller than the first predetermined number, and the computer program causes the image processing device to further execute a fourth step of determining a maximum timing that is a timing at which the CT value becomes a maximum value in a change-over-time of the CT value determined in the first step based on the first image data, and to execute the first step, the second step, and the third step based on the second image data including CT images in the second predetermined number of frames based on the maximum timing determined in the fourth step. to further execute a fourth step of determining a maximum timing that is a timing at which the CT value becomes a maximum value in a change-over-time of the CT value determined in the first step based on the first image data, and to execute the first step, the second step, and the third step based on the second image data including CT images in the second predetermined number of frames based on the maximum timing determined in the fourth step.