One aspect of the present invention is a signal detection device including a mode demultiplexer configured to convert N (N is a natural number) spatial mode signals including a plurality of higher modes excited in a transmission path fiber into N base modes and output the N basis modes, a frequency conversion unit configured to convert N optical signals into frequencies different from each other, a multiplexing unit configured to multiplex the N optical signals having frequencies that have been converted, and a reception unit configured to photoelectrically convert a multiplexed optical signal while maintaining modulation information for a phase and polarization, perform digital signal conversion and signal processing, and demodulate an original information sequence.