To provide a ternary judgement circuit for not causing ternary judgement errors even when noise provided with large waviness is added to reception signals in the ternary judgement circuit provided in the reception part of a communication system for performing communication by a digital cable transmission system by using ternary AMI signals.

Between an input terminal 11 and an output terminal 18, a switch 12, a first judgement device 13, a second judgement device 14, a memory control part 15, a memory 16 and a switch control part 17 are provided. In the case that signal data are the ternary AMI signals, in the second judgement device 14, the amplitude of the signal data in the past held in the memory and the ternary judgement result of the signal data are referred to and ternary judgement is performed while correcting noise components.