PURPOSE: To miniaturize the device, to make it inexpensive and to improve reliability by comparing the output digital signal of a digital signal demodulation means and the digital signal converted by a D/A conversion means and deciding whether they coincide or not.

CONSTITUTION: The analog voltage signal outputted by a D/A converter 33 is extracted in a pulse width modulation part 38, this is threshold processed by reference voltage and a pulse width modulation wave signal is inputted in a photocoupler 39. The signal is converted into the pulse width modulation wave of an electric signal and is outputted in a photo Tr 39b via a photodiode 39a, and it is inputted in a digital signal demodulation part 40, and the digital data corresponding to pulse width is written in a decision part 41. This and the data written from a control input part 31 are compared, whether they coincide or not is decided, and the result is outputted from a decision result output part 42. Therefore, reliability can be improved in probability because the D/A conversion processing of an insulated type D/A converter main body 33 can be confirmed, insulating an input/output, and a simple insulating constitution where an insulation is performed in photocouplers 35, 39 is made.