The present invention provides an optical signal quality monitoring circuit and an optical signal quality monitoring method for measuring correct optical signal quality parameters when a signal bit rate is changed. The optical signal quality monitoring circuit which samples and converts an electrical signal converted from an optical signal with a given repeated frequency f1 to digital sampling data through an analog to digital conversion, thereafter, evaluates an optical signal quality parameter of the optical signal by subjecting sampling data to electrical signal processing in an integrated circuit in which a signal processing function is programmed, receives a control signal notifying that the bit rate of the optical signal is changed, or detects that the bit rate of the optical signal is changed to correct optical the signal quality parameter of the optical signal corresponding to the signal bit rate of the optical signal which is changed.