PURPOSE: To eliminate data loss and the like at the time of demodulating an echo signal and to facilitate the detection of echo data.

CONSTITUTION: Command data which a command control circuit 1 generates are given to a command data editing circuit 2. The command data editing circuit 2 format-converts command data and outputs it as a digital command signal. The digital command signal is given to a command signal modulation circuit 3, is modulated and is outputted as a command signal. It is also given to an echo data detection circuit 4 as turn-back input. The echo data detection circuit 4 inversely format-converts turn-back input, namely, the digital command signal, detects it as echo data and outputs it to the command control circuit 1. The command control circuit 1 compares echo data with command data and collates echo.