PURPOSE: To improve the reliability of a sea water battery by placing a separator made of pulp which can be decomposed by water between the positive and the negative electrodes.

CONSTITUTION: A separator 4 which can be decomposed by water is made of pulp and prepared from paper. It is desirable that a binder or a hydration- resistant agent not be contained in the separator 4. The size of the separator 4 is adjusted to be either the same or as larger as possible than the size of the larger one of a positive and a negative electrode 2 and 3. The separator 4 is placed between the positive and the negative electrodes 2 and 3 in such a manner as to cover their facing surfaces. Accordingly, almost no short circuits develop during assembly of the sea water battery and therefore its reliability can be greatly improved.