An image processing apparatus includes a holder holding a carrier carrying cells, an imager including an imaging optical system in which an optical axis is oriented toward the carrier, and images the cells in a bright field, a position changer changing a focus position of the imaging optical system in a direction along the optical axis, a controller changing a set position of the focus position by the position changer and obtaining a plurality of original images by causing the imager to perform imaging at each set position, and an image processor calculating a contrast value of each original image, specifying two set positions at opposite sides of a local minimum value of the contrast value in a profile of the contrast value in relation to the set position and generating a difference image of two of the original images respectively imaged at the two set positions.