To provide an inexpensive gas leak alarm having an intruder detection function and an accompanying alarm output function.

The leak alarm 1A for outputting an abnormal gas concentration alarm is equipped with a person detection sensor 44 for detecting an intruder into a detection area, and outputs both an intruder alarm output when the intruder is detected, and an abnormal gas concentration alarm from a common alarm output part 23. The leak alarm is also equipped with a switch part 25 for switching the alarm output mode by the alarm output part 23 between a normal mode in which outputting an intruder alarm is prohibited, and outputting an abnormal gas concentration alarm is allowed, and an intruder monitoring mode in which outputting both alarms is allowed.