To allow a data communication equipment to quickly and effectively detect various errors in a transmission signal and to maintain high transmission quality.

This data communication equipment for a communication system that transmits data in terms of coded mark inversion(CMI) codes is provided with a clock-generating section 16 that uses a PLL to generate a system clock signal SCK phase-locked to a signal component, based on a clock signal component CK extracted by a CMI data signal and a fault detection section 17 that counts edges of the clock signal SCK for a period, when the CMI data signal has a prescribed signal level (e.g. CMI=1 level) and detects a fault of a transmission line when the count is a prescribed number or over, figures (A), (B), or a fault detection section 17 that counts edges of the CMI data signal for a period when the clock signal SCK has a prescribed signal level (e.g. SCK=1 level) and detects a fault of a transmission line, when the count is a prescribed number or over, figures (A), (C).