An image forming apparatus includes an observation image input section in which a plurality of observation images is input, an emphasis information input section that inputs information to be emphasized, a storage section that defines a plurality of conversion functions that converts the plurality of observation images into a converted image on the basis of a function for conversion and takes, as a parameter, a gradation value of each pixel in the plurality of observation images and a plurality of emphasis functions that takes, as a parameter, a gradation value of each pixel in the conversion functions, an image calculation section that calculates an image in which information to be emphasized is emphasized on the basis of the plurality of input observation images, the input information of the information to be emphasized, the conversion functions, and the emphasis functions, and an emphasized image output section that outputs the emphasized image.