The invention provides a light relay device and a light relay transmission system. In the light relay device carrying out relay transmission for the wavelength-division multiplexing light formed by carrying out wavelength-division multiplexing for multiple optical signals with different lengths, the optical power is maintained in a high precision and in a set range and a good transmission property is guaranteed, even on condition that loss of optical fibres as light transmission path is changed or wavelength-division multiplexing amount is changed. The invention comprises a total power detecting part (6) of wavelength-division multiplexing light used for detecting the total power of the wavelength-division multiplexing light; a wavelength-division multiplexing amount detecting part (5a) used for detecting the wavelength-division multiplexing amount of optical signals contained in the wavelength-division multiplexing light; a changeable attenuator (7) used for attenuating the total power of the wavelength-division multiplexing light; a control target value setting part (8) used for setting the control target value corresponding to the wavelength-division multiplexing amount according to the wavelength-division multiplexing amount detected by the wavelength-division multiplexing amount detecting part (5a); a control part (9) used for feeding back and controlling the changeable attenuator (7) according to the control target value so as to make the detected result of the total power of the wavelength-division multiplexing light consistent with the control target value; and an optical amplifier (4) used for amplifying light in a constant gain without relating to the power of the wavelength-division multiplexing light.