A method for discriminating a colony based on a photo image of the colony composed of pluripotent stem cells, the method comprising: discriminating between a differentiated colony containing differentiated pluripotent stem cells, an undifferentiated colony containing only undifferentiated pluripotent stem cells and a multilayered colony containing pluripotent stem cells stacked in multiple layers based on the luminance of the photo image, wherein: a colony having a region with a luminance higher than the first threshold of the luminance is determined as the differentiated colony; a colony having only a region with a luminance equal to or lower than the first threshold and equal to or higher than a second threshold is determined as the undifferentiated colony; and a colony having a region with a luminance lower than the second threshold is determined as the multilayered colony; and wherein the first thresholdis determined by applying a discriminant analysis to the histogram of luminance values from a median filtered image obtained by applying median filtering once or multiple times to the photo image; and the second threshold is determined by: obtaining an image of only the colony region by extracting, out of the pixels in the smoothing filtered image obtained by applying smoothing filtering once or multiple times to the photo image, only the pixels having a luminance value smaller than the threshold calculated by multiplying the first threshold by a predetermined multiplying factor in the range of from 105% to 115%; creating a histogram of luminance values from only the colony region, with the luminance value on the horizontal axis and the number of pixels having the corresponding luminance value on the vertical axis; determining a maximum luminance limit as the smaller value of the first threshold and the luminance value corresponding to the maximum value of the histogram, obtaining from the luminance value smaller than the maximum luminance limit, a straight line by the least squares method using the coordinates of the points on the histogram that lie between 20% and 90% of the number of pixels (the value of the vertical coordinate) corresponding to the maximum luminance limit; and determining as the second threshold the intersection point between the straight line obtained by the least squares method and the horizontal axis. a colony having a region with a luminance higher than the first threshold of the luminance is determined as the differentiated colony; a colony having only a region with a luminance equal to or lower than the first threshold and equal to or higher than a second threshold is determined as the undifferentiated colony; and a colony having a region with a luminance lower than the second threshold is determined as the multilayered colony; and wherein the first thresholdis determined by applying a discriminant analysis to the histogram of luminance values from a median filtered image obtained by applying median filtering once or multiple times to the photo image; and the second threshold is determined by: obtaining an image of only the colony region by extracting, out of the pixels in the smoothing filtered image obtained by applying smoothing filtering once or multiple times to the photo image, only the pixels having a luminance value smaller than the threshold calculated by multiplying the first threshold by a predetermined multiplying factor in the range of from 105% to 115%; creating a histogram of luminance values from only the colony region, with the luminance value on the horizontal axis and the number of pixels having the corresponding luminance value on the vertical axis; determining a maximum luminance limit as the smaller value of the first threshold and the luminance value corresponding to the maximum value of the histogram, obtaining from the luminance value smaller than the maximum luminance limit, a straight line by the least squares method using the coordinates of the points on the histogram that lie between 20% and 90% of the number of pixels (the value of the vertical coordinate) corresponding to the maximum luminance limit; and determining as the second threshold the intersection point between the straight line obtained by the least squares method and the horizontal axis.