PURPOSE: To provide a passive SSR device which can accurately calculate the position of a transponder on the two-dimensional plane using interrogation electric waves independently delivered by a plurality of SSR stations and also perform calculations for positions on the three-dimensional coordinate system as well as other advanced information.

CONSTITUTION: When an SSRA device wiretapps interrogation electric waves originated by an SSRB and corresponding response electric waves from a transponder S to detect the position of the transponder on the two-dimensional plain, the point of time when the rotating antenna of the SSRB faces the transponder S is detected based on the wiretapped interrogation and response electric waves to calculate the azimuth of the rotating antenna at that point of time to calculate the position of the transponder S on the two-dimensional plain. In addition, a propagation distance from the SSRB through the transponder S to the SSRA device is calculated to provided advanced information based on the position of the transponder on the two-dimensional plane as well as the propagation distance.