PURPOSE: To obtain an externally attaching type diagnostic apparatus which can be applied to various kinds of vehicles only by replacing adapters by providing a plurality of signal converting adapters by which the data code signals that are different for the kinds of vehicles are adapted to a data processing circuit so that the adapters can be freely attached and removed.

CONSTITUTION: A fault diagnosing apparatus 20 has a personal computer (PC) as a data processing circuit. Required signals are inputted through a touch keyboard 20a. Data code signals inputted from an automobile are processed in accordance with a program. The operating states of individual control objects 11a, 12a, 13a... are dis played on a display 20b. A signal conversion adapter 30 is attached to an input part 20c of the apparatus 20 so that the adapter can be freely attached and removed. A cord 22 having a connector 21 which is linked to the adapter 30 and a connector 23 which is connected to the automobile are provided. The adapter 30 has a converting circuit wherein the signal formats of the data code signals from computers 11, 12, 13... of the automobile 10 are converted so that the signal can be processed in the PC of the apparatus 20. In this way, the operating state of each equipment of the vehicle is diagnosed based on the data code signal of the control device inputted through the adapter 30.