For increasing needs of positioning technology, easy and simple indoor positioning system is desired. Wireless LAN based positioning system is a major candidate of indoor positioning technology. Conventional WLAN based systems require high installation cost or long site calibration before use. In this paper, we describe the implementation of simple WLAN based indoor positioning system WiPS. WiPS uses RSSI between terminals to avoid site calibration. We perform an experiment called a flag game, in which the person raises and lowers the PDA held in each hand. WiPS determines each hand is up or down. As results of the experiment, WiPS detects the movement of PDA in 4.8 sec, on an average.