An optical branching unit comprising: optical amplification means that, when an optical signal is received as input, amplifies and supplies the optical signal, or, when said optical signal is not received as input, amplifies and supplies an amplified spontaneous emission that the optical amplification means itself generates; detection means that detects whether said optical signal is applied as input to said optical amplification means; control means that, when said detection means detects that said optical signal is not applied as input to said optical amplification means, sets gain that determines the magnitude of amplification in said optical amplification means to a predetermined value that is greater than said gain when said optical signal is applied as input to said optical amplification means; add/drop multiplexing means that receives output light from said optical amplification means and other output light and that multiplexes an optical signal of a predetermined wavelength that is contained in the output light from said optical amplification means and an optical signal of a designated wavelength that is contained in said other output light; the branching unit characterized in that it further comprises designated optical amplification means that, when a designated optical signal is received as input, amplifies and supplies that designated optical signal as said other output light, or, when said designated optical signal is not received as input, amplifies and supplies an amplified spontaneous emission, which said designated optical amplification means itself generates, as said other output light; designated detection means that detects whether said designated optical signal is applied as input to said designated optical amplification means; and designated control means that, when said designated detection means detects that said designated optical signal is not applied as input to said designated optical amplification means, sets designated gain that determines the magnitude of amplification in said designated optical amplification means to a designated value that is greater than said designated gain at the time when said designated optical signal is applied as input to said designated optical amplification means, wherein a first block comprising said optical amplification means, said detection means and control means is placed in a trunk signal direction for detecting a trunk signal that is transmitted from a trunk station, wherein a second block comprising said designated optical amplification means, said designated detection means and said designated control means is placed in a branch signal direction for detecting a branch signal that is sent from a branch station. optical amplification means that, when an optical signal is received as input, amplifies and supplies the optical signal, or, when said optical signal is not received as input, amplifies and supplies an amplified spontaneous emission that the optical amplification means itself generates; detection means that detects whether said optical signal is applied as input to said optical amplification means; control means that, when said detection means detects that said optical signal is not applied as input to said optical amplification means, sets gain that determines the magnitude of amplification in said optical amplification means to a predetermined value that is greater than said gain when said optical signal is applied as input to said optical amplification means; add/drop multiplexing means that receives output light from said optical amplification means and other output light and that multiplexes an optical signal of a predetermined wavelength that is contained in the output light from said optical amplification means and an optical signal of a designated wavelength that is contained in said other output light; the branching unit characterized in that it further comprises designated optical amplification means that, when a designated optical signal is received as input, amplifies and supplies that designated optical signal as said other output light, or, when said designated optical signal is not received as input, amplifies and supplies an amplified spontaneous emission, which said designated optical amplification means itself generates, as said other output light; designated detection means that detects whether said designated optical signal is applied as input to said designated optical amplification means; and designated control means that, when said designated detection means detects that said designated optical signal is not applied as input to said designated optical amplification means, sets designated gain that determines the magnitude of amplification in said designated optical amplification means to a designated value that is greater than said designated gain at the time when said designated optical signal is applied as input to said designated optical amplification means,