A method for determining the state of a cell aggregation, characterized by the steps of: obtaining time lapse luminance images of a cell aggregation automatically taken by an imaging device (54c, 55c); running an image processing on the time lapse luminance images by segmenting cell aggregations in each of the time lapse luminance images, by labeling the segmented cell aggregations, and by making associations for the cell aggregations between the time lapse luminance images such that overlapping labels between time luminance images represent the same cell aggregation; aligning cell aggregations with the same label in order to reduce the effect of cases in which cell aggregations move; dividing time lapse luminance images of aligned and associated cell aggregations into a plurality of smaller regions as local regions being smaller than the size of cell aggregations and centered on pixels forming the time lapse luminance images; analyzing the time lapse luminance images by calculating a spatial change of pixel values for each of the small regions in each of the time lapse luminance images from the luminance distribution of the small region such that the spatial change is expressed quantitatively through a first score value (a); calculating a temporal change of pixel values of each of the small regions such that temporal change of pixel values is expressed quantitatively through a second score value (b); and determining a state of change in a small region of the cell aggregation toward becoming multi-layered when the related first score value (a) is at or above a first threshold value and a related second score value (b) is at or above a second threshold value. obtaining time lapse luminance images of a cell aggregation automatically taken by an imaging device (54c, 55c); running an image processing on the time lapse luminance images by segmenting cell aggregations in each of the time lapse luminance images, by labeling the segmented cell aggregations, and by making associations for the cell aggregations between the time lapse luminance images such that overlapping labels between time luminance images represent the same cell aggregation; aligning cell aggregations with the same label in order to reduce the effect of cases in which cell aggregations move; dividing time lapse luminance images of aligned and associated cell aggregations into a plurality of smaller regions as local regions being smaller than the size of cell aggregations and centered on pixels forming the time lapse luminance images; analyzing the time lapse luminance images by calculating a spatial change of pixel values for each of the small regions in each of the time lapse luminance images from the luminance distribution of the small region such that the spatial change is expressed quantitatively through a first score value (a); calculating a temporal change of pixel values of each of the small regions such that temporal change of pixel values is expressed quantitatively through a second score value (b); and determining a state of change in a small region of the cell aggregation toward becoming multi-layered when the related first score value (a) is at or above a first threshold value and a related second score value (b) is at or above a second threshold value.