1. An image processing apparatus comprising: a storage unit receiving a fluorescence observation image of a cell population stained by a fluorescence dye and a transillumination observation image having a same field of view as the fluorescence observation image; a processor configured to: specify, by applying a threshold value to the transillumination observation image, a first area group in which an image of the cell population exists in the transillumination observation image; set, as a reference area group, an area group corresponding to the first area group in the fluorescence observation image in which a mask is applied to the reference area group; and obtain fluorescence intensity data from the reference area group set in the fluorescence observation image.