An apparatus and method for individually detecting NO and NO 2 concentrations as NO X components of an object gas. An NO concentration corresponding value is obtained from a first detection element not having a reduction section. An NO concentration corresponding value is obtained from a second detection element having a reduction section, and having an NO 2 /NO sensitivity ratio greater than that of the first detection element. The difference ΔC between the NO concentration corresponding values of the two detection elements is obtained, and divided by the difference ΔS between the NO 2 /NO sensitivity ratios of the two detection elements, whereby an NO 2 concentration corresponding value is obtained. A value obtained by multiplying the NO 2 concentration corresponding value by the NO 2 /NO sensitivity ratio of the second detection element is subtracted from the NO concentration corresponding value of the second detection element, whereby an NO concentration corresponding value is obtained.