To suppress decomposition of lithium cobalt oxide and formation of a decomposition product. To suppress the reaction between oxygen in lithium cobalt oxide and a current collector. To obtain a power storage device having high charge and discharge capacity. In a method for manufacturing a power storage device, in forming a lithium cobalt oxide layer over a positive electrode current collector by a sputtering method using a target containing lithium cobalt oxide and a sputtering gas containing Ar, the positive electrode current collector is heated at a temperature at which c-axes of crystals of lithium cobalt oxide are aligned and cobalt oxide is not formed. The heating temperature of the positive electrode current collector is higher than or equal to 400° C. and lower than 600° C.