1. An image processing device comprising: a memory that stores a program; and a processor that executes the program stored in the memory, wherein the processor extracts a region of interest which includes a point of interest and satisfies a specified condition in a first image frame, the processor divides the region of interest into a first subregion including the point of interest and a second subregion not including the point of interest at a narrow portion of the region of interest, the processor specifies a specified pixel in the first subregion as a point of interest of a second image frame, and when the processor divides the region of interest into the first subregion and the second subregion, the processor repeatedly executes a contour searching process in one of a clockwise and counterclockwise direction, until a new contour is not searched, wherein in a first contour searching process, the processor searches an outer contour of the region of interest by extracting each pixel that is located on an edge of the region of interest started from a search start pixel that is determined based on a position of the point of interest and that is located on the edge of the region of interest, in each of following contour searching processes, the processor searches a next contour by extracting each pixel that is located adjacent inside a previous contour searched in an immediately previous contour searching process with at least a final contour searching process searching the first subregion without extending into the second subregion, and the processor detects the first subregion based on the position of the search start pixel for a most recently searched contour.