PURPOSE: To periodically measure the concns. of oxygen and hydrogen in gas to be measured, by periodically applying the first and second voltages by one limit current type oxygen sensor using a solid electrolyte.

CONSTITUTION: At first, the first voltage (low voltage) V1 is applied to an oxygen sensor and a stable time stabilizing the operation of the sensor is secured as a prescribed time to measure the limit current value IL1 of the first flat part of a voltage-current characteristics curve. This current value IL1 is stored in a memory and, at the same time, desired operation is performed to calculate the concn. of oxygen and the calculation result is outputted to a display to be displayed thereon. Next, the second voltage (high voltage) V2 is applied after the elapse of a predetermined time to measure the limit current value IL2 of the second flat part of the voltage-current characteristic curve. The difference ΔIL=IL2-IL1 between the output currents is calculated and the concn. of moisture (humidity) is operated from the difference current ΔIL and the operation result is outputted to the display to be displayed thereon. By this method, the concns. of oxygen and moisture in gas to be measured can be periodically measured by one limit current type oxygen sensor using a solid electrolyte.