JPH05332970

PURPOSE:To obtain a nitrogen oxide detecting sensor having a relatively simple and inexpensive structure, excellent selectivity for NOX and durability, by providing a gas detecting part of which the main constituent is a specific composite oxide and electrodes which are connected electrically with this detecting part. CONSTITUTION:A nitrogen oxide detecting sensor includes a gas detecting part 3 of which the main constituent is a composite oxide having electric conductivity or semiconductivity and expressed by a formula A1-XCaXB2M4O8+delta and of electrodes 4 and 5 which are connected electrically with the detecting part 3. In the formula, A is a group IIIa element, B is Sr, Ba or Ra; M is a transition metal element; (x) is 0 to 1; and delta is 0 to 0.4. This composite oxide has a layer- shaped oxygen lattice defect structure in a crystal structure and a nitrogen oxide molecule is made to coordinate with and eliminated from a portion of an oxygen defect reversibly. When this reaction occurs, the detecting part 3 itself shows a change in an electric resistance value in accordance with the concentration of a nitrogen oxide in a vapor phase. This change is detected by using the electrodes 4 and 5 and thereby the concentration of a nitrogen oxide gas contained in the air or a combustion exhaust gas can be detected with excellent selectivity.