Abstract: A quantum cryptography transmission system according to the present invention comprises a transmission device (10A), a reception device (20A), and a transmission path (30) configured to connect between the devices. The transmission device includes a light emitting unit (11) configured to emit photons serving as quantum bit information carriers, and a transmission-side optical circuit (12A). The reception device includes a light receiving unit (21) configured to detect photons serving as quantum bit information carriers, and a reception-side optical circuit (22A). Each of the transmission-side optical circuit (12A) and reception-side optical circuit (22A) is an optical circuit which is configured of an unbalanced Mach-Zehnder interferometer (123; 223) including an optical delay circuit (123-1; 223-1) in one arm thereof, and two transmission-side 3-dB couplers (126; 226) and (127; 227) to be connected to the two arms of the unbalanced Mach-Zehnder interferometer (123; 223), respectively.