A fertilized egg development stage determination method according to the invention includes: obtaining image data corresponding to an image of a fertilized egg captured by optical microscope; extracting a region corresponding to the fertilized egg in the image based on the image data and obtaining an index value indicating a degree of coincidence of the region with a circle; extracting a region corresponding to a zona pellucida of the fertilized egg in the image based on the image data and obtaining an index value indicating a thickness of the region; and determining a development stage of the fertilized egg based on the index value indicating the degree of coincidence, the index value indicating the thickness and determination criteria determined in advance for a combination of these index values.