PURPOSE: To improve the reliability of data communication and save the space for a bucket by conducting the operation control of the bucket by the cable-less optical data transmission between a transmitter/receiver provided facing vertically downward on the bucket side and a transmitter/receiver provided facing vertically upward on the ground side.

CONSTITUTION: Various data transmitted from a control board 12 are taken into the input circuit 41 of a ground side transmitter/receiver 10, subjected to modulation therein, and transmitted by a light emitting element 43. The light emitted from the light emitting element 43 is received by the light receiving element 51 of a transmitter/ receiver 4 on a bucket 2 side, demodulated by a demodulating circuit 52, and transmitted from an output circuit 52 to a control unit in an operation board 3 through an I/F 5. Various data formed in the operation board 3 are taken into the input circuit 56 of the transmitter/receiver 4, subjected to modulation therein, and transmitted by a light emitting element 54. The light emitted from the light emitting element 54 is received by the light receiving element 44 of the ground side transmitter/receiver 10, demodulated by a demodulating circuit 45, and transmitted from an output circuit 46 to a control unit in a control board 12 through an I/F 11.