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The paper presents the experimental results of comparison between quantum voltages at two Josephson-junction devices made from niobium and YBa2Cu3O7-δ high-temperature superconducting ceramics with the use of a SQUID galvanometer at 4,2 K. Voltage steps up to 290 μV at an irradiation frequency of 8,6 GHz have been investigated. It is found that the voltage step at ceramic junctions has zero slope and values of the constant KJ derived from the comparison of the two junctions are equal within a fractional measurement uncertainty 1 σ(KJ)/KJ of 5 × 10-8.