To provide ceramic electrets with superior biocompatibility and controlled surface energy utilizing long-distance ionic conduction under a DC electric field, the conduction property of B-type carbonated apatite (CA) ceramics with various carbonate content was investigated by complex impedance measurements and a concentration cell technique. Infrared spectroscopy and X-ray diffractometry indicated that B-type CA with the composition of Ca(10-x)Na(2x/3)(PO(4))(6-x)(CO(3))(x)(H(2)O)(x)(OH)(2-x/3) (x=ca. 0.87, 1.14, and 1.78, which is identified as CA1, CA2, and CA3, respectively) was prepared. The carbonate content significantly influenced the ionic conductivity (sigma) of the obtained CA, which drastically increased from 10(-8) to 10(-3) S/cm (700 degrees C) in the range between x = 0.87 and 1.14. Detection of stable electromotive forces in an oxygen concentration cell using CA1 and CA2 electrolyte (cathode: 5N-graded oxygen gas, anode: nitrogen- balanced 5%-95% oxygen gas) provided strong evidence for oxide ion conduction in B-type CA. Comparison of the obtained electromotive forces (EMF(ex)) with the theoretical ones (EMF(th)) suggested that the transference numbers for oxide ion conduction in CA1 and CA2 were 0.17 and 1.01, respectively. This result demonstrated that a substitution ratio in excess of one-sixth phosphate ions to carbonate ions (x = 1) converted the B-type CA from a proton conductor into an oxide ion conductor.