PURPOSE: To improve the convenience of tenants for access by counting infiltration monitoring signals outputted when the tenants are judged to have retreated from rooms of a building with a signal counting unit at a gateway and closing the door of the gateway when the number reaches the preset number.

CONSTITUTION: Retreat judging means are provided at rooms R1-Rn of a building with multiple tenants, and infiltration monitoring signals Sa1-San are outputted when the tenants are judged to have retreated. A signal counting unit is provided at the gateway 1 of the building, and the infiltration monitoring signals Sa1-San are counted. When the counted number reaches the preset number, all the tenants are judged to have retreated from their rooms, and the door of the gateway is closed thereafter for the preset period. The troublesome identification judgment at the gateway is not required when the tenants return to their rooms from the outside while the tenants are remaining in the building.