Provided are a signal-quality evaluation device and a signal adjustment method which require shorter evaluation time and which have high flexibility of application. A signal quality evaluation device 100 includes an optical component 110 and an optical output detector 120. In the optical component 110, an output optical power P.sub.out is a function of an input optical power P.sub.in, and this function P.sub.out(P.sub.in) has at least one maximum point. The optical output detector 120 detects the time-average power of light output from the optical component 110.