1. A particular-region detection method of detecting particular regions of persons from an image including a plurality of persons as objects by using a computer including calculating means and memory means, comprising: detecting a plurality of regions of the image satisfying detection conditions for particular region candidates as a particular region candidate by the calculating means of the computer; detecting a face region of a first person having a face in one of the plurality of regions including one of the particular region candidates and specifying the particular region candidate as a particular region to be detected on condition that the particular region candidate is included in the detected face region, by the calculating means of the computer; storing, by the memory means of the computer, face information regarding the face of the first person obtained when the face region is detected; and accessing the face information when a face region of a second person having a face different from the face of the first person is further detected in another region of the image including another particular region candidate after said storing of the face information, and detecting the face region of the second person by using the face information as a face detection condition, by the calculating means of the computer, wherein the first person face information comprises orientation information and face size information and the method further comprises: storing a plurality of predetermined face size values in a first sequence of a predetermined order; storing a plurality of predetermined face orientation values in a second sequence of a predetermined order; (a) determining the region which surrounds an exterior of the particular region candidate of the second person as lacking the face region, based on the face size and orientation information of the first person; (b) selecting a predetermined face size value from the first sequence; (c) determining whether the region which surrounds the exterior of the particular region candidate of the second person is the face region; and (d) based on step (c), one of: sequentially selecting predetermined face size values from the first sequence and repeating step (c) until all of the stored predetermined face size values are used, and detecting the second person face and storing face information of the second person.