A terminal multiplexer is constructed in such a manner that it can be upgraded for adapting to a change in a transmission system. Line terminating equipment (LTE) having working high speed transmission lines and protection high speed transmission lines in 1:1, is constructed from such units as two sets of 10G interfaces responsible for interface with the high speed transmission lines, and SELH which selects a high speed transmission line for which multiplex conversion is performed in relation to low speed transmission lines When this LTE is to be upgraded to ADM adapted for 2-Fiber BLSR, the unit SELH is replaced with SWH which performs switching of signals in each time slot between a high speed transmission line and the low speed transmission lines. Upgrading during the system operation is possible by making signal interfaces with the outside common for SELH and SWH, and by making delay times between signal input and output coincide for various signals.