The purpose of the present invention is to provide a composite sheet in which the dimension stability when it is tensed is high and the appearance deterioration and the yellowness change are suppressed. The present invention relates to a sheet comprising; cellulose fibers having a fiber width of 1000 nm or less and having a phosphoric acid group or a phosphoric acid group-derived substituent; and a polyvinyl alcohol-based resin, wherein the sheet has a tensile elastic modulus of 3.4 GPa or more.