PURPOSE: To prevent change of state of flip-flop etc. in a logical circuit device provided with a shift register that scans in and holds data for diagnosis by outputting a fixed signal from a selector during scanning in.

CONSTITUTION: The selector 1 receives the first select signal and selects data inputted from a data inputting terminal Din, receives the second select signal and selects data for diagnosis scanned in and held from a data for diagnosis inputting terminal TDin to a shift register 3, and receives the third select signal and selects fixed data preset in the circuit device and takes in the body 2 of the logical circuit device. As fixed data are outputted by giving the third select signal to the selector 1 when the data for diagnosis are scanned in the shift register 3, change of state of flip-flop etc. of the body of the logical circuit device is prevented.