A differential signaling system, wherein a first wiring and a second wiring are coupled between a sending end and a receiving end as a differential signal line. A termination resistor is coupled between the first wiring and the second wiring in the receiving end side. A test circuit is coupled to the termination resistor in parallel, and amplifies and detects a variation of a differential impedance due to the differential signal line. The test circuit includes: a differential test amplifier for amplifying a variation in the differential impedance of the first wiring or the second wiring; a switching unit installed at an input terminal of the differential test amplifier for controlling an operation of the differential test amplifier; and a peak detector for converting an output signal of the differential test amplifier into a direct current component; and a phase detector for detecting a skew, a time delay, and/or a phase difference of a signal inputted to the differential signal line.