To enable mounting and demounting of a ground pickup irrespective of a connection box by dividing a connection cable into a ground pickup side cable connected to a ground pickup and a connection box side cable connected to a connection box, and connecting them by a detachable connector.

A ground pickup side connection cable L1 is connected to a connection box side cable L2 connected to a water resistant connection box 2a provided in correspondence with a ground pickup 1a. The cable L2 is connected to a terminal board 3a provided for the box 2a. The connector 10a is composed of a detachable connector and capable of connecting or disconnecting the cable L1 composed of a plurality of cables to or from the cable L2 provided corresponding to the number of the cable L1. Accordingly, each of ground pickups 1a-1b can be removed freely by the connectors 10a-10c.