PURPOSE: To lower the load of a biological treatment apparatus by eliminating the loss of sugar content by a method in which a diluted sugar sirup is treated by a two-stage osmotic membrane apparatus so as to obtain a comparatively thicker sugar sirup and a purified water containing little sugar.

CONSTITUTION: The diluted sugar sirup 1 stored in a storage tank A is passed through the first stage-osmotic membrane apparatus B using a tubular type osmotic membrane with a 90% or more desalting capability under pressure in order to concentrate the sugar, whereby obtaining a concentrated sugar sirup 2 and the primary permeated water 3. The concentrated sugar sirup 2 is circulated to the storage tank A, and at a time when its concentration reaches a given one, the sugar sirup is sent to the outside of the system for recovery. On the other hand, the primary permeated water 3 is stored in a storage tank C, and then passed through the second stage- osmotic membrane apparatus D using a coiled or hollow fiber type osmotic membrane with a 90% desalting capability under a pressurized condition in order to obtain a purified permeated water 4 and a concentrated water 5. The purified permeated water 4, having the composition of nearly pure water, is recovered and used for various purposes. The concentrated water 5 is recycled to the storage tank A or the storage tank C, where sugar is again recovered.