1. An initialization method to be used in a network in which plural stages of data transmission apparatuses, each performing data transmission while assigning one or more bits of data as one data symbol to a signal level, are connected in a ring shape, wherein one of the data transmission apparatuses is a master data transmission apparatus, while the other data transmission apparatuses are slave data transmission apparatuses; at turn-on of power or immediately after reset, the master data transmission apparatus outputs a first initialization pattern signal to a slave data transmission apparatus in the next stage; the next-stage slave data transmission apparatus establishes clock synchronization on receipt of the first initialization pattern signal; when clock synchronization is established, the next-stage slave data transmission apparatus outputs the first initialization pattern signal to a slave data transmission apparatus in the next stage; when synchronization is established in all of the slave data transmission apparatuses and the master data transmission apparatus which are connected in the network, the master data transmission apparatus outputs a second initialization pattern signal to the next-stage slave data transmission apparatus; the next-stage slave data transmission apparatus receives the second initialization pattern signal, sets an evaluation level for each signal level, and outputs the second initialization pattern to a data transmission apparatus in the next stage; and transmission/reception of data is started after evaluation levels of data are set in all of the slave data transmission apparatuses and the master data transmission apparatus which are connected in the network.