An optical amplifying unit that enhances the flexibility of the installation within the system is disclosed. The optical amplifying unit includes a semiconductor optical amplifier (SOA) and a housing that encloses the SOA. The housing provides a front wall with a front window that passes an optical axis coming from the SOA. The optical amplifying unit further provides a front coupling unit directly fixed to the front wall without interposing any optical fibers. The optical coupling unit has the optical receptacle function to pluggably receive an external optical plug.