Abstract: A device for determining an alert condition in a system for a gate which device is usable easily in any building and is suitable for a building whose security is not strict, is provided. The system 1 includes: an access device 4, a gate 2 being opened or opening of the gate 2 being permitted at least after the access device 4 is accessed; a full closing detector 3 detecting full closing of the gate 2; an object detector 5 including at least one sensor 5a or 5b detecting an object in a detection area 50a or 50b which is near the gate 2 and within at least one of two zones B and C; and the device 6. The device 6 is connected to the detectors 3 and 5, and determines the alert condition after a predetermined combination of full-closing detection or non-detection and object detection or non-detection occurs.