Each cell is pressurized with gas from outside of the cell, the amount of permeation of the gas permeated into each cell is measured, and a cell having the amount of permeation greater than (average value of all cells+A) (wherein A is a predetermined value of σ to 6σ, where σ is the standard deviation) is considered to be defective. Alternatively, pressure is reduced for each cell, the degree of vacuum in each cell is measured, and a cell having the degree of vacuum worse than (average value of all cells+A) is considered to be defective. Then, a polymer compound is poured into the defective cells of the monolithic separation membrane structure and cured so that the defective cells are sealed. Alternatively, the polymer compound formed in advance as the sealing member is inserted into the defective cells to seal the defective cells.