In order to monitor various types of noises which are to be introduced on signals through signal lines on a circuit board and automatically adjust the thresholds for signal state discriminations to make it possible to surely make a signal state discrimination without being affected by these noises even if the amplitude of a signal is reduced for higher-speed transmission and lowered electric power, there is provided a configuration comprising a signal generation unit generating a noise monitor signal; a noise monitor signal line receiving and propagating the noise monitor signal; a noise detection unit detecting a noise which has been introduced into that noise monitor signal propagated through the noise monitor signal line and which affects a state discrimination using a threshold; and a threshold adjustment unit, if the noise detection unit detects the noise, adjusting the threshold such that the state discrimination is not affected by the noise.