The present invention provides an electrochemical device which is reduced in the thickness to the level where the electrochemical device is able to be incorporated in a thin electronic device such asan IC card; and a method for manufacturing this electrochemical device. An electrochemical device which comprises: an element main body (10) wherein a pair of internal electrodes (16, 26) are laminated so as to sandwich a separator sheet (11) therebetween; an outer casing sheet (4) that covers the element main body (10); sealing parts (40, 42) that hermetically seal the peripheral part of the outer casing sheet (4) so that the element main body (10) is immersed in an electrolyte solution; and lead terminals (18, 28) that are electrically connected to the internal electrodes (16, 26) respectively and are led out from the sealing parts (40, 42) of the outer casing sheet (4) to the outside. A resin inner layer (4B) that is present in the inner surface of the outer casing sheet (4) is providedwith a fusion bonding part (4B1) for separators, which partially bonds a part of the separator sheet (11) to the inner surface of the outer casing sheet (4).