A person is detected by an observer device. A person detection history is recorded in a person detection history database. A position where the person is detected for the first time is estimated by a first-time detection position estimating unit, to be stored in a first-time detection position history database. The position of the door of the room in terms of local coordinates is estimated by a doorway position estimating unit. The calibration information as to the position of the observer device is calculated by a position calibration information calculating unit based on the door position in terms of local coordinates and that in terms of global coordinates.