SrTiO3 ceramic capacitors were studied to improve the temperature coefficient, tau, of dielectric constants without decreasing the dielectric constants. It was indicated that an addition of calcium in an insulating reagent improved tau. From scanning electron microscopy and energy-dispersive X-ray analysis measurements, the origin of this improvement was ascribed to the following layers : (1) the oxide layers which were formed at the grain boundaries, and (2) the bismuth-oxide layers which were formed on the sample surface.