An accuracy of the sex determination which is visually inspected by human based on a reference drawing of an egg shape prepared for every parent chicken is insufficient and it takes much time to perform such a determination. Thus, the sex of the fertilized egg is determined by using parameters representing features developed on a surface shape of the above described fertilized egg which are quantified based on contour image data of the above described fertilized egg obtained by taking an image of the fertilized egg which is placed on a setting stand.