A digital signal processing device comprising: a first digital signal processing section including a first signal processing section and first and second input/output interface sections (A, B); and a second digital signal processing section including a second signal processing section, third and fourth input/output interface sections (C, D), and a selection section, wherein the first and second signal processing sections (6a-6b) selectively switch modulation/demodulation in low-efficiency modulation system and modulation/demodulation in high-efficiency modulation system which transmits and receives an optical signal at a bit rate that is plural times of a bit rate of the low-efficiency modulation system, and perform digital signal processing, the first, second, third and fourth input/output interface sections (A-D) mutually convert serial signals and parallel signals, parallel-side interfaces of the first and second input/output interface sections (A, B) are electrically connected to the first signal processing section, a serial-side interface of the second input/output interface section is electrically connected to a serial-side interface of the fourth input/output interface section, the selection section electrically connects a parallel-side interface of the third input/output interface section to the second signal processing section when the low-efficiency modulation system is selected, and the selection section electrically connects the parallel-side interface of the third input/output interface section to a parallel-side interface of the fourth input/output interface section when the high-efficiency modulation system is selected. a first digital signal processing section including a first signal processing section and first and second input/output interface sections (A, B); and a second digital signal processing section including a second signal processing section, third and fourth input/output interface sections (C, D), and a selection section, wherein the first and second signal processing sections (6a-6b) selectively switch modulation/demodulation in low-efficiency modulation system and modulation/demodulation in high-efficiency modulation system which transmits and receives an optical signal at a bit rate that is plural times of a bit rate of the low-efficiency modulation system, and perform digital signal processing, the first, second, third and fourth input/output interface sections (A-D) mutually convert serial signals and parallel signals, parallel-side interfaces of the first and second input/output interface sections (A, B) are electrically connected to the first signal processing section, a serial-side interface of the second input/output interface section is electrically connected to a serial-side interface of the fourth input/output interface section, the selection section electrically connects a parallel-side interface of the third input/output interface section to the second signal processing section when the low-efficiency modulation system is selected, and the selection section electrically connects the parallel-side interface of the third input/output interface section to a parallel-side interface of the fourth input/output interface section when the high-efficiency modulation system is selected.