1. A technique for determining the state of a cell aggregation, comprising: obtaining time lapse images of a cell aggregation taken by an imaging device; calculating a spatial distribution of pixel values and a temporal change of pixel values of a region in the cell aggregation in the obtained images; and determining a state of change of the cell aggregation toward becoming multi-layered on the basis of the calculated spatial distribution, the calculated time lapse change of the pixel values and at least one threshold value.