The invention provides a communication device of an operation machine, wherein the communication device can realize high safety and quick operation response. The communication device is provided with the components of an upper transmitting receiver which transmits an operation signal that corresponds with the operation of an upper operation device as a wireless signal; and a lower transmitting receiver which can receive the operation signal from the upper transmitting receiver and transmits an operation instruction based on the operation signal to a lower controller. The lower transmitting receiver transmits an ACK signal back to the upper transmitting receiver and the wireless signal on the condition that the operation signal from the upper operation device is received, and furthermore transmits the operation instruction based on the operation signal to the lower controller. The upper transmitting receiver determines whether the operation instruction requires receiving confirmation according to the content of the operation instruction. Furthermore on the condition that receiving confirmation is required, the operation signal is transmitted to the lower transmitting receiver again when the ACK signal of the operation signal is not transmitted back from the lower transmitting receiver.