A working machine comprising: a machine body (2, 3); a working device which includes a working device main body (41, 42) mounted on the machine body and an attachment detachable from the working device main body (41, 42), the working device being operable to displace the attachment relatively to the machine body; a transmitter disposed in the attachment so as to be displaced together with the attachment relatively to the machine body and configured to send an information signal containing information on the attachment; a receiver disposed in the machine body (2, 3) and being capable of receiving the information signal; a controller which acquires information on the attachment based on the information signal received by the receiver; and a relay device disposed in the working device main body (41, 42) to receive the information signal sent by the transmitter and to transmit the information signal to a position at which the information signal can be received by the receiver, wherein the relay device includes at least one relay unit (81, 82), the at least one relay unit (81, 82) including a relay reception antenna capable of receiving the information signal, a relay transmission antenna capable of sending the information signal, and a transmission line which interconnects the relay reception antenna and the relay transmission antenna so as to transmit the information signal received by the relay reception antenna to the relay transmission antenna, the at least one relay unit (81, 82) thus being configured to receive the information signal and send the received information signal to allow the information signal to be transmitted from the transmitter to the receiver, characterized in that the at least one relay unit (81, 82) includes a plurality of relay units, the plurality of relay units including: a transmission-side relay unit located closest to the transmitter of the plurality of relay units, the transmission-side relay unit being disposed at such a position that the information signal sent by the transmitter can be received by the relay reception antenna of the transmission-side relay unit, and a reception-side relay unit located closest to the receiver of the plurality of relay units, the reception-side relay unit being disposed at such a position that the information signal sent from the relay transmission antenna of the reception-side relay unit can be received by the receiver, the information signal sent from the relay transmission antenna of the transmission-side relay unit being transmitted to the relay reception antenna of the reception-side relay unit directly or via other relay unit. a machine body (2, 3); a working device which includes a working device main body (41, 42) mounted on the machine body and an attachment detachable from the working device main body (41, 42), the working device being operable to displace the attachment relatively to the machine body; a transmitter disposed in the attachment so as to be displaced together with the attachment relatively to the machine body and configured to send an information signal containing information on the attachment; a receiver disposed in the machine body (2, 3) and being capable of receiving the information signal; a controller which acquires information on the attachment based on the information signal received by the receiver; and a relay device disposed in the working device main body (41, 42) to receive the information signal sent by the transmitter and to transmit the information signal to a position at which the information signal can be received by the receiver, wherein the relay device includes at least one relay unit (81, 82), the at least one relay unit (81, 82) including a relay reception antenna capable of receiving the information signal, a relay transmission antenna capable of sending the information signal, and a transmission line which interconnects the relay reception antenna and the relay transmission antenna so as to transmit the information signal received by the relay reception antenna to the relay transmission antenna, the at least one relay unit (81, 82) thus being configured to receive the information signal and send the received information signal to allow the information signal to be transmitted from the transmitter to the receiver, characterized in that the at least one relay unit (81, 82) includes a plurality of relay units, the plurality of relay units including: a transmission-side relay unit located closest to the transmitter of the plurality of relay units, the transmission-side relay unit being disposed at such a position that the information signal sent by the transmitter can be received by the relay reception antenna of the transmission-side relay unit, and a reception-side relay unit located closest to the receiver of the plurality of relay units, the reception-side relay unit being disposed at such a position that the information signal sent from the relay transmission antenna of the reception-side relay unit can be received by the receiver, the information signal sent from the relay transmission antenna of the transmission-side relay unit being transmitted to the relay reception antenna of the reception-side relay unit directly or via other relay unit.