1. A method for region segmentation of urine particle images, comprising the steps of: extracting a first object region larger than an object particle included therein, from a urine particle image taken by an image input optical system, by using at least one of an image with red components of the urine particle image (an R image), an image with green components of the urine particle image (a G image), and an image with blue components of the urine particle image (a B image); calculating a size of the first object region based on at least one of an area or a perimeter of the first object region and a density distribution of the first object region, in the first object region, of each of the at least one of the R image, the G image, and the B image, and classifying the first object region into one group of a predetermined number of groups, based on the size of the first object region and the density distribution of the first object region; and extracting a second object region, representing the object particle, from the first object region, by using group-specific region segmentation, which includes use of the at least one of the R image, the G image, and the B image.