To provide a mechanism for preventing being locked up in a trunk capable of effectively and reliably preventing an infant, etc., from being locked up.

In a motor vehicle provided with a trunk room to be closed by a lock means when a lid is closed, a lock detecting means (latch switch) 5 to detect the operation of the lock means and a human detecting means (trunk opening part sensor) 4 to detect the presence of a human are installed in the trunk room, and alarming means (hazard lamp 11 and horn 12) to issue alarms by the operation of turning on both detecting means 5 and 4 are provided. In case that the trunk lid is closed and locked with an infant, etc., mistakenly locked up in the trunk room, it is possible for an adult, etc., present nearby to notice the accident as the horn 12 is sounded simultaneously with the flashing of the hazard lamp 11, which are the alarming means.