PURPOSE: To simply and automatically measure whether or not each of terminal equipments is connected to signal lines interconnecting a DSU in an ISDN and each terminal equipment in a correct polarity.

CONSTITUTION: Each violation of 1st and 2nd polarity sides of a pulse signal subject to AMI coding is detected by 1st and 2nd violation detection sections 15a, 15b. After the 1st violation detection section 15a detects violation, before a prescribed bit depending on a specified frame structure of the pulse signal elapses, when a violation detection signal is outputted from any violation detection section, the 1st polarity is discriminated to be the positive polarity. The detector is provided with a polarity discrimination section 18 in which when the detection signal is not outputted, the 2nd polarity is discriminated to be the positive polarity.