PURPOSE: To secure a sheet of separation paper capable of easily meeting the promotion of automatic tube-making for a separator in an inexpensive and easy manner, by mixing a highly pure natural cellulose fiber being digested and more than one type of combined fibers in an undefined direction, while forming it into a sheetlike substance.

CONSTITUTION: A highly pure natural cellulose fiber and more than one type of combined fibers of 3μ∼15μ in diameter each are mixed together with a weight percentage of 85:15∼15:85. A more desirable compounding ratio is in a range of 60:40∼40:60. This natural cellulose fiber conduces to liquid keeping capacity for an electrolyte, while the combined fiber contributes to an alkaliproof strength holding function preventing deterioration due to an alkaline electrolyte during application from occurring. At the time of securing a sheet of separation paper after mixture, it is desirable that each fiber be disposed in an undefined direction and made into a sheeted form, and in this connection, if they are disposed in a fixed direction, for example, a vertical or horizontal direction so excessively, a reinforcing aggregate effect in the alkaline electrolyte is lowered down to some extent.