A composite body includes a substrate containing Cr; and a first composite oxide layer disposed on at least a part of a surface of the substrate, the first composite oxide layer having a spinel type crystal structure, a first largest content and a second largest content among constituent elements excluding oxygen of the first composite oxide layer being Zn and Al in random order. Accordingly, the composite body can suppress diffusion of Cr from the substrate containing Cr to the first composite oxide layer, and has improved long-term reliability. A collector member and a gas tank, each of which is formed of the composite body, can have improved long-term reliability. A fuel cell device having excellent long-term reliability can be obtained using the collector member and the gas tank.