To provide a bias supply device, capable of surely suppressing oscillations over a broad band, by certainly preventing connection between the inlet side and outlet side of a tested device, without affecting the existing measurement system.

The bias supply device 5 comprises bias terminals 11S1 and 11S2 for receiving bias voltages S1 and S2, ground terminals 11G1 and 11G2 for receiving ground potentials G1 and G2, output terminals 121 and 122 for outputting the bias voltages S1 and S2 and the ground potentials G1 and G2 to the tested device, being mutually shielded four signal lines for connecting the bias terminals 11S1 and 11S2 and ground terminals 11G1 and 11G2 to the output terminals 121 and 122, and reactance elements 141, 142, 151, 152, 161, 162, 171, 172, having a closed magnetic circuit structure inserted into each of these signal lines.