An optical signal quality monitor includes a splitter splitting an input optical signal into two signals; a low-frequency converter converting one split optical signal to a low frequency signal by modulating the optical signal with a frequency offset signal; and an intensity ratio calculator calculating an intensity ratio between the low frequency signal and the other split optical signal, thereby appropriately confirming the quality of a high-bit rate optical signal. The monitor includes plural processing lines, each line including the splitter, the low-frequency converter, and the intensity ratio calculator. At least one line includes an optical noise superimposer superimposing optical noise on the one split signal before inputted to the converter or an optical band-pass filter transmitting the one split signal before inputted to the converter. The monitor includes a polarization state changer changing the polarization state of the input signal before inputted to the splitter.