In a secondary battery including a positive electrode, a negative electrode and a porous film bonded to the surface of at least one of the positive electrode and the negative electrode, the porous film includes ceramic particles and a binder, and the ceramic particles include polycrystalline particles obtained by mechanically crushing a fired material comprising a ceramic that is directly synthesized from a ceramic precursor. The porous film has a porosity of 40 to 80%, for example. The porous film can be formed by a method including the steps of: obtaining a fired material comprising a ceramic from a ceramic precursor; obtaining ceramic particles by mechanically crushing the fired material of the ceramic; obtaining a slurry including the ceramic particles and a binder; and applying the slurry onto the surface of an electrode, followed by drying.