A multiplexing device including a multistage slave configured to process control data transmitted from a master in an industrial network, a multiprocessing device configured to multiplex the control data transmitted from a first slave of the multistage slave, the first slave being disposed at an upstream side to the master of the multistage slave, and transmit the control data so multiplexed to a second slave of the multistage slave by way of a multiplex communication line, the second slave being situated at a downstream side of the multistage slave, and a control device configured to issue a disconnection command to disconnect a communication between the first slave and the multiprocessing device in response to a disconnection of the multiplex communication line.