Bonding agent

Powders of respective metal elements (Mn, Co) constituting a transition metal oxide (MnCo 2 O 4 ) having a spinel type crystal structure are used as a starting material. A paste containing the mixture of the powders is interposed between an air electrode and an interconnector, and with this state, a sintering is performed, whereby a bonding agent according to the present invention can be obtained. This bonding agent has a "co-continuous structure". In the "co-continuous structure", a thickness of an arm portion that links many base portions to one another is 0.3 to 2.5 µm. The bonding agent includes a spherical particle in which plural crystal faces are exposed to the surface, the particle having a side with a length of 1 µm or more, among the plural sides constituting the outline of the crystal face. The diameter of the particle is 5 to 80 µm.