1. A person recognition apparatus comprising: a facial feature information storage that stores therein facial feature information of a person by each person; and at least one processor that receives image data captured by a camera; that detects a face region representing a face of a person from the received image data; that extracts facial feature information indicating a feature of the face of the person from the detected face region; that selects one of an operation mode and a setting mode, wherein in the operation mode, it is determined whether the person captured by the camera is a registered person who has been preregistered, and in the setting mode, a threshold value to be used upon the determination is set; that determines, as the registered person and when the operation mode is selected, a person with a degree of similarity between the extracted facial feature information and the stored facial feature information of greater than or equal to the threshold value; that outputs a determination result of the at least one processor; and that sets, when the setting mode is selected, the threshold value based on first degrees of similarity and second degrees of similarity, wherein the first degrees of similarity are degrees of similarity between facial feature information extracted by capturing the registered person with the camera and the stored facial feature information of the registered person captured, the second degrees of similarity are degrees of similarity between facial feature information extracted by capturing a person, the facial feature information of the person not previously stored, other than the registered person with the camera and the stored facial feature information, in the setting mode, the at least one processor selects one of a registrant walking mode and a non-registrant walking mode, when the registrant walking mode is selected, the at least one processor selects at least a lower one of the first degrees of similarity, and generates a first distribution of similarity based on the first degrees of similarity, and when the non-registrant walking mode is selected, the at least one processor selects at least a higher one of the second degrees of similarity, and generates a second distribution of similarity based on the second degrees of similarity; the at least one processor sets, based on the first and second distributions of similarity, the threshold value so that at least one of a rejection rate and an acceptance rate becomes a preset value, the rejection rate being a rate at which the at least one processor does not determine the registered person as a registered person, the acceptance rate is a rate at which the at least one processor determines the person other than the registered person as a registered person.