To provide a monitor circuit for one-wire communication capable of easily specifying a fault source by an extremely simple circuit.

A monitor circuit for one-wire communication includes a level holding circuit 71 which is equipped with a pair of monitor terminals 73 and 74 connected between single communication lines 51 interconnected to a plurality of communication means by a half-duplex communication method to discriminate the transmission side or the reception side of a signal transmitted by the communication lines, and holds an active signal level transmitted to the transmission-side monitor terminal 73 (74) and also holds a non-active signal level before transmission to the reception-side monitor terminal 74 (73); and a signal transmission circuit 72 which transmits the active signal level from the transmission side to the reception side.