Disclosed are optical signal transmission apparatus including a reflective gain-clamped semiconductor optical amplifier and optical communication system using the optical signal transmission apparatus, which can improve modulation speed and optical power by effectively suppressing intensity noise of an incoherent light source. The optical signal transmission apparatus comprises a light source; a reflective gain-clamped semiconductor optical amplifier to generate gain-clamped optical signals having a substantially constant output intensity in a gain saturation region; a wavelength division multiplexing apparatus configured to spectrum-slice light from the light source, provide the spectrum-sliced light to the reflective gain-clamped semiconductor optical amplifier, and multiplex optical signals gain-clamped by the reflective gain-clamped optical amplifier, and a circulator for inputting the light generated by the light source to the wavelength division multiplexing apparatus, and outputting the optical signal multiplexed by the wavelength division multiplexing apparatus to a transmission link.