A first electrode and a second electrode to be used are electrodes each of which has a collector, and a porous material layer with electron conductivity placed between the collector and a separator, and each of which has a configuration wherein the porous material layer includes at least particles of a porous material with electron conductivity, and a thermoplastic resin being capable of binding the particles of the porous material together and having a softening point T.sub.B lower than a softening point T.sub.S of the separator. A production method includes a thermal treatment step of thermally treating a laminate at a thermal treatment temperature T1 satisfying a condition represented by Formula (1): T.sub.B.ltoreq.T1