An intrusion detection device to detect an intrusion of an object within a warning area, which warning area is variable in a direction and which intrusion detection device is formed by an optical section, including a detecting element, and a light collecting section, the device comprising: an area adjusting unit provided in one of the optical section and the light collecting section to optically adjust an optical path between the optical and light collecting sections; and a position setting unit provided in the other of the optical section and the light collecting section to define a position of the area adjusting unit when the position setting unit is engaged with the area adjusting unit; wherein the warning area has a plurality of narrow width areas and the area adjusting unit optically adjusts the optical path which forms the respective narrow width area; the area adjusting unit is a variable shutter to optically block the optical path forming at least one of the narrow width areas and the position setting unit is a position setting rib to perform a position setting when the position setting unit is engaged with the variable shutter; the optical section comprises a main unit, having the detecting element, and a unit holder to pivotally support the main unit, the variable shutter being mounted on the main unit; the light collecting section comprises an array of segmentalized lenses formed in a covering which covers the main unit, the position setting rib being mounted on the covering; and the main unit and the covering being configured such that, when the main unit is set at a predetermined pivot position and, when the covering is fitted to the main unit, it engages the position setting rib, which is mounted on the covering, to set the position of the variable shutter. an area adjusting unit provided in one of the optical section and the light collecting section to optically adjust an optical path between the optical and light collecting sections; and a position setting unit provided in the other of the optical section and the light collecting section to define a position of the area adjusting unit when the position setting unit is engaged with the area adjusting unit; wherein the warning area has a plurality of narrow width areas and the area adjusting unit optically adjusts the optical path which forms the respective narrow width area; the area adjusting unit is a variable shutter to optically block the optical path forming at least one of the narrow width areas and the position setting unit is a position setting rib to perform a position setting when the position setting unit is engaged with the variable shutter; the optical section comprises a main unit, having the detecting element, and a unit holder to pivotally support the main unit, the variable shutter being mounted on the main unit; the light collecting section comprises an array of segmentalized lenses formed in a covering which covers the main unit, the position setting rib being mounted on the covering; and the main unit and the covering being configured such that, when the main unit is set at a predetermined pivot position and, when the covering is fitted to the main unit, it engages the position setting rib, which is mounted on the covering, to set the position of the variable shutter.