Radiation properties of magnetic noise from the harness wires of a spacecraft (GEOTAIL) have been studied experimentally and theoretically. A simulation experiment on the noise radiation using a minimum set of subsystems of the spacecraft has shown that the intensity and the directional patterns of the noise radiation from the wires were largely changed by the existence of a conductive plate near the harness wires. The change in the noise characteristics is explained by eddy currents induced in the conductive plate by the signal current flowing in the wires. The eddy currents distributed in the conductive plate were calculated by the Finite Element analysis Method (FEM). The magnetic flux densities calculated from both the source signal current and its induced eddy currents for the wiring configuration of the simulation experiment have shown to be consistent with the values obtained in the experiment. The results in the present study have provided us an important information on a wiring method to diminish noise radiation from harness wires.