Information from one or more transmitters installed in a detection object area is received. The reception feature value of when a detection object is present in a predetermined position of the detection object area is measured. A reception pattern of when the detection object is present in the predetermined position is formed. The reception pattern is compared with reference reception patterns, and the presence position corresponding to the reference reception pattern most approximate to the reception pattern is acquired from a reference reception pattern database where the presence positions of the detection object and the reference reception patterns of when the detection object is present in the presence positions are associated with one another. The presence position of the detection object is estimated according to the acquired presence position. The estimated presence position of the detection object is associated with received detection object identification information.