1. A method for constructing a cell lineage comprising the steps of: (a) obtaining a plurality of 2D images different in focal plane and a time by taking a plurality of the 2D images by changing the focal plane and taking a plurality of the 2D images in a time series for a cell constituting the subject of observation; (b) extracting a nucleus area by carrying out image processing for individual 2D images; (c) unifying the nucleus area, which is derived from the identical nucleus, from the nucleus area extracted from the individual 2D images; and (d) constructing the cell lineage from a time and a position, where the nucleus area emerges and disappears in the image, in the unified nucleus area.