To independently adjust each CN (Carrier to Noise)ratio of a transmitting signal and an up-convert transmitting signal to simultaneously maximize both the CN ratios in an optical fiber transmitting device applying optical fiber transmission to a multiple signal containing the transmitting signal, separating the multiple signal into the transmitting signal and the up-convert transmitting signal formed by up-converting the transmitting signal at a light reception side.

The optical fiber transmitting device comprises: branching a carrier wave A from a laser light source 1 into two at an optical coupler 2; inputting one branched carrier wave A1 to a first light modulator 3 for modulation by the transmitting signal B, generating a light modulating signal C; inputting the other branched carrier wave A2 to a second light modulator 4 for modulation by a high-frequency signal D, generating a light sub-carrier wave F; multiplexing the light modulating signal C and the light sub-carrier wave F to generate a light multiple signal G for the optical fiber transmission. The light multiple signal G is separated into an electric signal I corresponding to the transmission signal B and an electrical signal J corresponding to the up-converted transmitting signal B by a branching filter 10 at the light reception side.