A solid electrolyte assembly includes a substrate, a solid electrolyte, and a first electrode and a second electrode. The solid electrolyte has oxide ion conductivity, and the first electrode is located under the solid electrolyte and contains a porous material. The first electrode has only one oxygen diffusion path that is formed overlapping the substrate and the solid electrolyte. When Sr represents the smallest cross-sectional area that is a cross-sectional area of the oxygen diffusion path at a location where the area of a cross section of the first electrode taken perpendicularly to an oxygen diffusing direction is smallest in a portion of the oxygen diffusion path where the oxygen diffusion path overlaps the substrate and the solid electrolyte in a plan view of the solid electrolyte assembly, and Sp represents the area of a region where the first electrode and the second electrode overlap each other in a plan view of the solid electrolyte, Sr/Sp is from 1×10−7 to 6.9×10−4.