Polycrystalline samples of perovskite Ba1-xKxK xBiO3 with a wide range of x have successfully been synthesized in a quite short heating time of several minutes at a temperature as low as 290°C using molten KOH. The powder X-ray diffraction patterns have revealed the formation of well-crystallized Ba1-xK xBiO3. The maximum value of the superconducting transition temperature Is ∼30 K observed at x ∼ 0.34 in the magnetic susceptibility measurements. The superconducting volume fraction drastically decreases for x < 0.37. These results are in good agreement with those of samples obtained by the conventional solid-state reaction method.