Proton conductor, fuel-cell solid-electrolyte layer, cell structure, and fuel cell provided with same

A solid electrolyte layer according to the present invention contains a proton conductor that has a perovskite structure and is represented by the following formula (1): Ba x Zr y Ce z M 1-(y+z) O 3-δ (where element M is at least an element selected from the group consisting of Y, Yb, Er, Ho, Tm, Gd, and Sc, 0.85≤x<0.98, 0.70≤y+z<1.00, the ratio y/z is 0.5/0.5 to 1/0, and δ is the amount of oxygen deficiency).