A first gas sensing element is provided in a first position to measure a concentration of a gas continuously, and a second sensing element is provided in a second position to measure the concentration of the gas intermittently. The second position is in close proximity to the first position. A deterioration determination unit is connected with the first and second sensing elements to make a determination as to whether the first sensing element becomes deteriorated, by comparing output values from the first and second sensing elements. A power controller is configured to supply the second sensing element with electric power throughout waiting periods during which the second sensing element stops measuring the concentration of the gas, and an amount of the electric power supplied throughout waiting periods is smaller than that supplied throughout operating periods during which the second sensing element continuously measures the concentration of the gas.