Abstract: A time division duplex (TDD) portable telephone relay device (and TDD system employing the same) for use with a radio base station and a portable telephone terminal within a terminal building, includes first and second distributors, a synthesizer, a predetermined-size (e.g., relatively small, internal) base station section, a position detecting unit, a synchronous receive section, and an exclusive telephone line connecting unit. With such a structure, transmission/reception using a frequency channel, other than frequency channels assigned to a radio zone where the terminal building exists, is conducted between the portable telephone terminal within the terminal building and the radio base station through the predetermined-size base station section. The predetermined-size base station section executes the operation adapted to TDD based on a synchronous signal from the synchronous receive section.