PURPOSE: To detect concentration in a broad range from a rich value to a lean value by specifying the ratio between the area of the atmosphere introducing hole of a heater and the area of the outer electrode of a detecting element at a specified value.

CONSTITUTION: A zirconia detecting element 1 is contained in a housing 3. A ceramic heater 2 is arranged at the inner side of the element 1. A platinum outer electrode 9a and an inner electrode 9b are formed for the element 1. A diffused layer 8 is formed on the outer electrode 9a. The heating part of the heater 2 is arranged so as to agree with the electrode 9a. Positive and negative power supplies are connected to the electrodes 9b and 9a, respectively. In this constitution, atmosphere which is introduced into the inside of the element 1 through the atmosphere introducing port of the heater 2 becomes oxygen ions at the time of rich measurement, and pumping is performed form the inner electrode 9b to the outer electrode 9a. At the time of lean measurement, pumping is performed from the outside to the inside. When the amount of the atmosphere required for the outer electrode 9a is made to be 10 times or less with respect to the amount of the atmosphere introduced through the atmosphere introducing port of the heat 2, the atmosphere required for the rich measurement is introduced. Thus, the measurement in the broad range from rich to lean can be performed.