PURPOSE: To obtain a monitor with simple constitution, easy production and adjustment, and high sensitivity by converging infrared rays from plural convex lenses on the photodetecting face of a single infrared detecting element by optical fibers.

CONSTITUTION: The convex lenses 1W4 are set so as to form monitoring visual fields 6W9 having prescribed intervals respectively. One end parts of the optical fibers 10W13 are arranged on the other focus parts of these respective convex lenses 1W4 and these are held in a body by frames 14W17 for holding and the other end parts are converged to become one fiber and arranged on the photodetecting face of the infrared detecting element 5. Further, the respective frames 14W17 for holding are divided respectively and the monitoring visual fields 6W9 can be made adjustable optionally. When the human body is passed through the inside of such monitoring visual fields 6W9, emitted infrared rays are made incident on the respective convex lenses 1W4 in order or some of these convex lenses and detected by a detecting circuit via the optical fibers 10W13 and alarmed or displayed.