Optical signal quality monitoring is an important function for optical transport networks and future all-optical networks. To monitor the optical signal-to-noise ratio and/or waveform distortion transparently with respect to the signal format, data format, and signal bit rate, we introduce an optical signal quality monitoring method that uses asynchronous sampling, which is a sampling technique that does not use timing extraction. The use of high-speed asynchronous sampling and the adjustment of the sampling rate enable simple open eye-diagram monitoring and evaluation of a fixed-timing Q-factor (Qt) at the maximum eye opening timing phase. This method was experimentally verified using an optical signal quality monitoring circuit, and obtained a good relationship between the measured Qt and Q (which is a Q-factor calculated from the bit error rate (BER)). Moreover, we also introduce an average Q-factor (Qavg) evaluation method, which measures the Qavg value from an asynchronous eye-diagram (timing drifted eye-diagram). This method is useful when the sampling rate is low or when adjusting the sampling rate is difficult.