An apparatus and a method for monitoring in-band OSNR (Optical Signal-to-Noise Ratio) which monitors the in-band OSNR by using two parallel Mach-Zehnder-interferometers with different optical time delays are disclosed. The apparatus and method can be resistant to chromatic dispersion, polarization mode dispersion and polarized noise, can measure the coherence characteristics of the signal without removing the noise, and can be manufactured into a semiconductor integrated device and be applied in the future high-speed optical network.