A computer-implemented image processing method for specifying a boundary between an invalid area in a peripheral part of a recess of a container containing a culture medium and a valid area in a central part of the recess from an image obtained by imaging the recess, the image processing method comprising: defining a plurality of search areas (R0-R7) of a predetermined size having mutually different circumferential positions along the peripheral part, each of the search areas extending in a radial direction from an origin defined in the central part of the recess toward the peripheral part, wherein the plurality of search areas are equidistant from the origin and are defined at equal angular intervals in the circumferential direction; executing (S104-S105) an edge detection process in each search area to obtain edge position and edge intensity of an edge in each search area; for each search area, obtaining (S106-S107) a relative shift amount such that, when another search area adjacent to the search area in the circumferential direction is shifted in the radial direction as much as the relative shift amount, a similarity between the search area and the other search area is maximized; and specifying a position of the boundary in each search area based on the edge positions, the edge intensities and the relative shift amounts obtained in the search area and each of other search areas in neighboring ranges in the circumferential direction around the search area, wherein for each search area, the position of the boundary in the search area is defined as: if an edge having a larger edge intensity than a predetermined value is detected in the search area, the edge position of the edge; and otherwise, a position obtained by correcting the position of the boundary in the other search area adjacent in the circumferential direction according to the relative shift amount of the search area. defining a plurality of search areas (R0-R7) of a predetermined size having mutually different circumferential positions along the peripheral part, each of the search areas extending in a radial direction from an origin defined in the central part of the recess toward the peripheral part, wherein the plurality of search areas are equidistant from the origin and are defined at equal angular intervals in the circumferential direction; executing (S104-S105) an edge detection process in each search area to obtain edge position and edge intensity of an edge in each search area; for each search area, obtaining (S106-S107) a relative shift amount such that, when another search area adjacent to the search area in the circumferential direction is shifted in the radial direction as much as the relative shift amount, a similarity between the search area and the other search area is maximized; and specifying a position of the boundary in each search area based on the edge positions, the edge intensities and the relative shift amounts obtained in the search area and each of other search areas in neighboring ranges in the circumferential direction around the search area, wherein for each search area, the position of the boundary in the search area is defined as: if an edge having a larger edge intensity than a predetermined value is detected in the search area, the edge position of the edge; and otherwise, a position obtained by correcting the position of the boundary in the other search area adjacent in the circumferential direction according to the relative shift amount of the search area. if an edge having a larger edge intensity than a predetermined value is detected in the search area, the edge position of the edge; and otherwise, a position obtained by correcting the position of the boundary in the other search area adjacent in the circumferential direction according to the relative shift amount of the search area.