An optical signal quality monitoring apparatus includes an optical detector for directly receiving an optical signal modulated in an optical path and converting the optical signal to an electric signal, an asynchronous sampling unit for asynchronously sampling the electric signal of the optical detector at a reduced speed, and a digital signal processor for monitoring an optical signal quality by finding a synchronized amplitude histogram of data sampled in the asynchronous sampling unit. An optical signal quality monitoring method includes (a) a step of allowing an optical detector to directly receive a modulated optical signal and to convert the optical signal to an electric signal; (b) a step of allowing an asynchronous sampling unit to asynchronously sample the electric signal; and (c) a step of allowing a digital signal processor to monitor an optical signal quality by generating a synchronized amplitude histogram of sampled data.