a composite oxide, as a main component thereof, represented by the following composition formula (1) or (2);

Cu as a first additive, in a content α, in terms of Cu₂O, falling within a range 0<α≤0.5% by mass in relation to said main component;

at least one selected from Dy, Nd, Eu, Gd, Tb, Ho and Er, as a second additive, in a content β, in terms of oxide, falling within a range 0<β≤0.3% by mass, in relation to said main component; and

at least one selected from Ta, Nb, W and Sb, as a third additive, in a content γ, in terms of oxide, falling within a range 0<γ≤0.6% by mass in relation to said main component;

wherein:

Pb_a[(Zn_1/3Nb_2/3)_xTi_yZr_z]O₃,  composition formula (1):

 a, x, y and z in the composition formula (1) satisfying the following relations, 0.96≤a≤1.03, 0.005≤x≤0.047, 0.42≤y≤0.53, 0.45≤z≤0.56, and x+y+z=1; and

(Pb_a−bMe_b)[(Zn_1/3Nb_2/3)_xTi_yZr_z]O₃,  composition formula (2):

 a, b, x, y and z in the composition formula (2) satisfying the following relations, 0.96≤a≤1.03, 0<b≤0.1, 0.005≤x≤0.047, 0.42≤y≤0.53, 0.45≤z≤0.56, and x+y+z=1, and Me in the composition formula (2) representing at least one selected from Sr, Ca and Ba.