It is generally known that the electric field intensity of radio signals changes according to the distance between the radio transmitter and radio receiver.  Using this matter, techniques of calculating the distances between a radio transmitter and radio receivers based on the electric field intensity of radio signals received by the radio receivers installed at different locations and locating the radio transmitter transmitting the radio signals using the principle of so-called triangulation are generally known. 
The above locating techniques work on the presumption that there are no people and/or objects (obstacles) that hamper the radio wave propagation in the space where the locating takes place.  In other words, when there are obstacles, the radio transmitter may not be accurately located because the radio signals are affected by multipath fading and the like. 
The locating apparatus of Patent Literature 1 is known to be able to accurately locate the radio transmitter even under the circumstances in which there are obstacles.  In this locating apparatus, information representing the electric field intensity of received radio signals transmitted from a given location is recorded for each divided area of a space in advance.  This locating apparatus makes reference to the information recorded in advance based on the electric field intensity of radio signals a communication terminal actually moving in the space has received at a given location in the space to locate the communication terminal. 
Patent Literature 1: Unexamined Japanese Patent Application Kokai Publication No.  H11-178045. 
When the spatial radio wave propagation environment is subject to change as a result of some obstacle having moved or the like, the electric field intensity of radio signals received by the communication terminal placed in each area also changes; then, the locating technique described in the Patent Literature 1 fails to locate the communication device with accuracy.