To easily perform an inspection without removing a measuring instrument to be inspected from a panel by inserting a connecting cable for inspection between signal transmitting side and receiving side through a connector, and engagingly locking the connector of an inspecting machine to the other end of the cable in inspection.

Connectors 4, 5 of a connecting cable line 8 for inspection are engagingly locked to signal transmitting side and receiving side connectors 2, 3, respectively, and a connector 6 on the other end of the cable line 8 is protruded onto the front panel of a driver's seat, and engagingly locked and connected to a conducting connector 7 by an internal wiring. In the inspection of a measuring instrument such as tachograph 1 or the like, when the connector 7 is removed from the connector 6, and the connector 10 of a tachograph inspecting machine 9 is connected to the connector 6, the signal from each sensor is interrupted, and only a power source is supplied to the tachograph 7 to form a state where the signal from the inspecting machine 9 can be supplied. The signal equal to in normal operation of each sensor is supplied from the inspecting machine 9 to the tachograph 1, and its measuring instrument display or recording state is confirmed to inspect whether the measuring instrument is normal or not.