PURPOSE: To cancel an offset amount without the need of a special circuit on an outside by caneling the offset amount from AD converted digital data by a software processing.

CONSTITUTION: At the time of shifting to a normal communication mode through the training mode of an isolated pulse, a signal input means 1 performs the input processing of input signals and a mode changeover means 2 outputs input data at the time of the training mode. A data judgement means 3 judges the input data and a data fetching means 4 fetches the input data on a positive side and a negative side among the input data as offset data corresponding to the judged result. An offset calculating means 5 calculates the offset amount based on the fetched offset data and an offset amount storage means 6 stores the offset amount. In the communication mode, a storage amount is subtracted from the input data outputted from the mode changeover means 2 by an offset cancellation means 7.