Provided is a solid electrolyte sheet having a self-supporting property while having a small thickness and flexibility. The solid electrolyte sheet is formed using a support having a specific porosity and a specific thickness. Specifically, the solid electrolyte sheet is formed in which a solid electrolyte is filled in a support having a porosity of 60% or more and 95% or less and a thickness of 5 μm or more and less than 20 μm.