1. An image processor configured to quantify a specific biological material in a region of interest of a sample stained with a first staining reagent capable of staining the specific biological material and stained with a second staining reagent capable of staining a specific region of a cell without interfering with staining with the first staining reagent, the image processor comprising: first input means configured to input a first image that shows a position of expression of the biological material stained with the first staining reagent; second input means configured to input a second image that is acquired in the same field of view as the first image and shows a morphology of the specific region of the cell stained with the second staining reagent; second image specific region extraction means configured to extract, as a second image specific region, the specific region from the second image; derivation means configured to derive the region of interest based on at least the second image having undergone extraction of the specific region among the first image and the second image having undergone extraction of the specific region; and density calculation means configured to calculate a density of the specific region, wherein the derivation means is configured to perform dilation for dilating the specific region and to derive the dilated specific region as the region of interest, and wherein the derivation means is configured to determine a dilation ratio for the dilation based on the density.