To provide a sighting device for use in a security sensor system, in which device enables an operator, assigned to conduct a sighting work, to look into the sighting device from a proper direction. The sighting device includes an eyepiece lens provided with a microlens, an objective lens provided with a first marker encompassed within the field of view of the eyepiece lens, and a reflecting mirror disposed on an optical path between the eyepiece lens and the objective lens. The first marker is comprised of a circular contour line or a polygonal contour line and is so set that when a viewing axis offsets from a tolerance, the first marker is viewed with a part thereof dropped out having been offset from the field of view of the microlens.