PURPOSE: To accurately display diagnostic data by automating the editing of said data, by providing an information register holding annexed informations such as a register name, a bit length or the like in a logical circuit having a scanning path.

CONSTITUTION: A logical circuit 1 is constituted so that usual registers 15, 16 performing usual logical operation and information registers 17, 18 preliminarily holding annexed informations such as data name information A, bit length information B or the like stored therein are connected in series to form a scanning path. The editing part 21 of a diagnostic data display circuit 2 picks up not only a signal name from the information A of the register 17 but also a state value led out in succession to the bit length of the register 15 from the information B. A display part 22 displays the signal name and state value picked up as a pair. The register 16 is operated according to the same procedure. As mentioned above, by providing the information registers corresponding to the usual registers to perform scanning path connection, the editing of diagnostic data is automated and accurate display can be performed.