PURPOSE: To make the polarity control of a line unnecessary by storing the output of a feeding polarity detecting means in a specified state of the line, and detecting the feeding polarity of the line at the optional state of the line by referring to information stored in an storage means.

CONSTITUTION: In the case that the lines L1, L2 are connected as shown by a full line, a first polarity detection circuit DETA operates as the detection circuit of normal polarity, and in the case that the lines L1, L2 are connected as shown by a broken line, the detection circuit DETA operates as the detection circuit of reverse polarity. Since the feeding polarity to be impressed to the lines L1, L2 is either the normal polarity or the reverse polarity, while the detection circuit DETA operates as the detection circuit of the normal polarity, if no signal appears in its output OA, it comes to be detecting the reverse polarity. On the contrary, while the detection circuit DETA operates as the detection circuit of the reverse polarity, if no signal appears in its output OA, it comes to be detecting the normal polarity.