A method for trimming a rectangular container and an apparatus trimming a rectangular container are provided, by which an appropriate cut surface with a possible crack or burr suppressed can be obtained when a rectangular container is trimmed. When a punch cutter is initially allowed to cut into a rectangular tubular portion of an untrimmed rectangular container, the punch cutter is inclined relative to a thickness direction of a cut area of the rectangular tubular portion, and then, the punch cutter is circumferentially moved along a trim line on the rectangular tubular portion to cut the rectangular tubular portion.