A disclosed composite structure includes a base material (X) and a layer (Y). The layer (Y) includes a mixture of a metal oxide (A), a phosphorus compound (B), and a compound (La) (silicon compound). A compound containing a site capable of reacting with the metal oxide (A) is included in the phosphorus compound (B) and compound (La) examples. The equation 0.01<=NSi/NM<=0.30 is satisfied when the number of moles of a metal atom (M) derived from the metal oxide (A) is NM and the number of moles of a Si atom derived from the compound (La) is NSi. In addition, the equation 0.8<=NM/NP<=4.5 is satisfied when the number of moles of a phosphorus atom derived from the phosphorus compound (B) is NP.