PURPOSE: To judge the active state of an oxygen concentration sensor accurately, by detecting the fact that the voltages between the electrodes of a battery element and an oxygen pump element are the voltages within the specified ranges after supply of a pump current to the oxygen pump element is started.

CONSTITUTION: A detecting part 1 of an oxygen concentration sensor, which is arranged in the exhaust system of an internal combustion engine 2, is composed of an oxygen-ion conductive solid-state electrolyte material 12 and pairs of electrodes 16a, 16b, 17a and 17b, which hold the material. The solid-state electrolyte material 12 and the pair of the electrodes 16a and 16b act as an oxygen pump element 18. The solid-state electrolyte material 12 and the pair of the electrodes 17a and 17b act as a battery element 19. A heater element 20, which heats the elements 18 and 19, is provided on the outer wall surface of an atmosphere reference chamber 15. Supply of a pump current to the element is started based on the control of a control part 4. When the voltage between the electrodes of the element is the voltage within a first specified range and the voltage between the electrodes between the electrodes of the element 18 is the voltage within a second specified range, it is assumed that the temperature of each element has reached a temperature, where the desired proportional output characteristic is obtained. Thus the active state of the oxygen concentration sensor is judged.