To obtain a transponder which can easily detect leakage of liquid from a distant place, its antenna structure, and a transponder unit.

A transponder 10 is constituted by sandwiching a transponder element 11 and an antenna 12, formed spirally of a belt-like conductor whose width is set extremely larger than the thickness between insulating film 13 and 14. Consequently, when an object body of detection such as water and oil comes into contact, the object body of detection generates electrostatic capacity between the adjacent belt-like conductors constituting the antenna 12 and a resonance frequency and transmission frequency of the antenna shift higher or lowers from its ordinary frequencies; and a matching state between the antenna 12 and transponder element 11 is deteriorates, and an arrival distance of a radio wave radiated from the antenna becomes short, so that even when a question signal is sent to the transponder, an answer signal from the transponder can no longer be received.