TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The method for preparing the pyrochlore-type oxide involves: preparing a first aqueous solution, which is an aqueous solution of the halide or nitrate of the A, and a second aqueous solution, which is an aqueous solution of the alkaline metal acid of the B'; forming a precipitate of the pyrochlore-type oxide by carrying out the reaction by dropping one aqueous solution, which is either the first aqueous solution or the second aqueous solution in the other; and calcining the precipitate of the pyrochlore-type oxide at a temperature of at least 250 degrees C to at most 900 degrees C. The method for preparing the pyrochlore-type oxide further involves before the forming of the precipitate of the pyrochlore-type oxide by the dropping, dispersing a conductive material in advance in either the first aqueous solution or the second aqueous solution. The reaction temperature during the forming of the precipitate is at least 0 degrees C to at most 60 degrees C. The calcining of the precipitate of the pyrochlore-type oxide is carried out in an inert gas atmosphere. In the method, A(NO3)2 is used as a precursor of A, and K2BO4 or Na2BO4 is used as a precursor of the B'. The A(NO3)2 contains at least Pb(NO3)2 or Sn(NO3)2, the K2BO4 is K2RuO4, and the Na2BO4 is Na2RuO4. The method (M2) involves preparing a first aqueous solution, which is an aqueous solution of the halide or nitrate of the A, and a second aqueous solution, which is an aqueous solution of the alkaline metal acid of the B'; forming a precipitate of the pyrochlore-type oxide by carrying out the reaction by dropping one aqueous solution, which is either the first aqueous solution or the second aqueous solution, in the other; and calcining the precipitate of the pyrochlore-type oxide at a temperature of at least 250 degrees C to at most 900 degrees C. The method (M2) further involves before the forming of the precipitate of the pyrochlore-type oxide by the dropping, dispersing a conductive material in advance in either the first aqueous solution or the second aqueous solution. Preferred Definitions:    A=a metal A2, which is different from A1;    B=a metal B2, which is different from B1;    A2 and B2=at least one element selected from Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Os, Co, Rh, Ni, Pd, Cu, Al, Ga, In, Ge, As, Sb, Bi, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu.    EXAMPLE - Preparation of pyrochlore-type oxide Pb2Ru2O7-z was carried out as follows: a 0.5 mol/l lead(II) nitrate aqueous solution obtained by dissolving lead(II) nitrate (Pb(NO3)2) in purified water was poured into a beaker. While stirring the aqueous solution at room temperature (about 25 degrees C), a 0.5 mol/l potassium ruthenate aqueous solution obtained by dissolving potassium ruthenate (K2RuO4) in purified water was dropped so that the molar ratio of Pb to Ru was 1 to 1. Then, a precipitate produced in the beaker was filtered off and washed with purified water, and a produced pyrochlore-type oxide was taken out and dried for 12 hours at 120 degrees C in flowing air. The LOI of a product after the drying was 3.5%, and the BET specific surface area was 69 m2/g. The product was transferred in a porcelain crucible and calcined in air in a muffle furnace. The calcination was carried out at 150 degrees C for 1 hour, and the calcination temperature was then raised at a rate of 5 degrees C/minute to 400 degrees C and kept at the temperature for 6 hours. After the temperature was lowered to the room temperature, the product was taken out. The BET specific surface area was 62 m2/g, and a peak particular to a pyrochlore lattice was observed in XRD. A peak half-width obtained based on the measurement and an average particle size that could be obtained by the Scherrer's equation were approximately 10 nm. Thus, it was confirmed that almost no particle aggregation occurred due to the calcination.