A vehicle height sensor includes a switching circuit which is turned on and off in accordance with a vehicle height. When a drive signal is outputted to the vehicle height sensor, the vehicle height sensor is activated. A connector is connected to an output side of the switching circuit. A digital vehicle height data signal is transmitted from the switching circuit via the connector. A preset signal is outputted from the vehicle height sensor to the connector when a check signal is generated. The preset signal represents preset check data. A data receives the preset signal via the connector and reads the check data from the preset signal. The read check data is compared with a predetermined reference data. The connector is judged to be wrong when the read check data differs from the reference data.