A sealing technique is established in which a seal can be easily formed and which is excellent in reliability and a heat cycle property in a high temperature region of 800.degree. C. or higher, so as to provide a composite body preferably used for a device for producing pure oxygen, oxygen-rich air, and the like, a membrane reactor represented by that for partial oxidation of a hydrocarbon gas, a solid oxide fuel cell, an oxygen purification device, a heat exchanger, or the like. The present invention makes it possible to increase a possibility of practical use in a wide area which has been delayed in development owing to a bottleneck of improvement in a sealing property. Particularly, its application to the device for producing pure oxygen, oxygen-rich air, or the like, the membrane reactor represented by that for partial oxidation of the hydrocarbon gas, the solid oxide fuel cell, the oxygen purification device, the heat exchanger, or the like can greatly contribute to acceleration of the development.