Provided are a multiplex communication system and a work machine that can appropriately perform transmission and reception of data between a processing circuit used in an industrial network and a multiplex processing section that multiplexes and transmits control data transmitted in the industrial network. A slave processes control data in the industrial network. The control data processed by the slave is transmitted and received by the multiplex communication by a multiplex processing section. A DUMMY-PHY, which is connected between the multiplex processing section and the slave, generates, in a pseudo manner, a signal compliant with an MII communication standard using a pseudo signal generation section, and transmits the generated signal to the slave, thereby performing establishment of communication. The DUMMY-PHY performs data transmission between the multiplex processing section and the slave after establishing communication with the slave.