1. An image analysis method comprising: using a computer to perform the following operations of: inputting time-series image data; generating time-series object image data by extracting object regions from the time-series image data, wherein the object regions are extracted such that differences between frames of the time-series image data are computed, and regions of large differences are digitized; computing a position of a center of gravity in each of the object regions, to thereby compute multiple respective centers of gravity; synthesizing, for each of the multiple centers of gravity, a respective additional line indicative of the position of a respective of the multiple centers of gravity, with the object image data; and analyzing motion of an object from the time-series object image data.