PURPOSE: To mount the processing unit with a high density and miniaturization by separating completely an analog section and a digital section electrically and in a structural way.

CONSTITUTION: The processing unit consists of an analog board 20 on which analog components of an analog input part 1 and an analog output part 5 only are integrated, an AD/DA board 21 on which components of an A/D converter part 15 and a D/A converter part 16 only are integrated, and a digital board on which digital components of a digital signal processing part 3 only are integrated. Then the analog board 20 and the digital board 22 are connected to the AD/DA board 21, and analog components and digital components are coupled via the AD/DA board 21 so as to completely separate the analog section and the digital section.