1. A masquerading detection device which, for use in determining whether a target is a specified person or not, detects the target being a non-specified person who masquerades as said specified person, comprising: target information obtaining means having a light emitter which irradiates said target with light, as well as projecting a predetermined pattern onto the target, a filter which transmits light emitted by the light emitter disposed in the vicinity of a light source of said light emitter, and target image obtaining means which photographs a target on which a predetermined pattern is projected by light emission of said light emitter to obtain target image data indicative of the target, image storage means which stores said target image data, and data processing means having masquerading determination means which determines whether said target is said non-specified person or not, wherein said masquerading determination means compares a pattern appearing in a face region of a target image represented by said target image data and said predetermined pattern to determine whether said target has a plane configuration or not and when the target has a plane configuration, determines that the target is a non-specified person: wherein said data processing means includes face detection means which detects a face region of a target image represented by target image data, and said masquerading determination means determines whether a target has a plane configuration or not by using a pattern appearing in a face region detected by said face detection means.