In order to provide an extended branch device in which construction work is easy and communication is not significantly affected by construction work, and a method for controlling the extended branch device, the extended branch device of the present invention is provided with: a first branch unit provided with a first port coupled to a first terminal station, a second port coupled to a second terminal station, a third port, a fourth port, and a switch for coupling the first port with the second or third port and coupling the second port with the fourth port; and a first separation unit provided with a fifth port coupled to the third port, a sixth port coupled to the fourth port, and a seventh port coupled to a third terminal station, the first separation unit outputting, from the sixth port, an optical signal having a first wavelength among the optical signals inputted from the fifth port, and outputting, from the seventh port, an optical signal having a second wavelength among the optical signals inputted from the fifth port. The extended branch device is further provided with a second branch unit configured so as to be separable from the first branch unit.