A signal quality detector circuit which detects an error signal of an input signal, used for detecting transmission quality of a transmission line. The detector circuit equalizes the input baseband signal, analog-to-digital-converts the equalized signal to a digital bit signal, analog-to-digital converts the input baseband signal to a digital bit signal, compares the two analog-to-digital converted signals on a bit-by-bit basis and detects an error signal accumulates the error signal in a unit time, and detects transmission quality of a transmission line.