An electrochemical element comprises a container and a sealing plate for sealing an opening of the container. The container is sealed with the sealing plate by welding a portion of the brazing material on the sealing plate and a portion of the metal film of the container which make contact with each other together in a state where a first electrode active material, a separator, a second electrode active material and an electrolyte are contained in the container. An electricity collector sheet is electrically connected to the first electrode active material or the second electrode active material, whichever is closer to the opening of the container. At least a portion of the electricity collector sheet extends to the end portion of the container so that the container and the sealing plate are welded together with the extending portion of the electricity collector sheet in between. Thus, the manufacturing process may be simplified so that the welding of the container and the sealing plate can be easily carried out even when the container and the sealing plate are reduced in size.