1. An apparatus for detecting entering and/or leaving comprising: a position measurement reliability detector that detects a position measurement reliability of a position measurer; a possible existing region determiner that determines a possible existing region in which a target subjected to a position measurement by the position measurer might exist based on a result of the position measurement which is obtained by the position measurer and the position measurement reliability obtained by the position measurement reliability detector; and a decider that decides entering and/or leaving of the target into and/or from a specific area based on an overlap of the possible existing region and the specific area.