The invention relates to a system for monitoring a person in a building, in which there is at least one room (H1, H2, etc.) to be monitored. It comprises more than one detector (T1, T2) in the room for registering detection data and sending it by wireless or wired technology to an electronics unit (E) belonging to the system, the detectors are set on the walls of the room in their lower parts extending to the floor level, in such a way that some of them (T1 ) at least on one specific wall (S1 ) and some of them (T2) on a wall (S2) adjacent to the said specific wall, that is at a different direction to it, and each of them have a band to be monitored of a certain width and each of these bands is delimited horizontally by the bands adjacent to them, so that the detectors are able to detect detection data appearing in the horizontally penetrating bands crossing each other in the room, and a monitoring grid is thus formed in the room, the detection data appearing in each square of which is detected simultaneously by at least two detectors detecting in different directions, and the monitoring grid is formed to be a certain height (h) and the electronics unit (E) receives detection data sent from the room through the detectors and processes it itself, or sends it to a monitoring system (V) belonging to the said system to be processed.
