A capacitor which comprises: an electrode group that is provided with a positive electrode having a positive electrode active material and a porous positive electrode collector which supports the positive electrode active material, a negative electrode having a negative electrode active material and a porous negative electrode collector which supports the negative electrode active material, and a first separator interposed between the positive electrode and the negative electrode; a nonaqueous electrolyte having alkali metal ion conductivity; a case that hermetically seals the electrode group and the nonaqueous electrolyte; an alkali metal supply source interposed between the electrode group and the case; and a second separator interposed between the electrode group and the alkali metal supply source. At least the negative electrode contains an alkali metal predoped from the alkali metal supply source. The second separator has a thickness of 5-60 [mu]m. The negative electrode collector contains a first metal porous body having a three-dimensional network structure.