A method for manufacturing a supporting plate provided inside of a disc-type filter having filter media at surface portions thereof including the steps of: forming an annular plate having opposed surfaces; and forming a plurality of wave portions on the annular plate that alternately extend beyond the opposed surfaces in opposite directions by pressing, each of the plurality of wave portions extending substantially in a circumferential direction of the annular plate and the plurality of wave portions being arranged substantially adjacent to each other in a radial direction of the annular plate. The supporting plate has high pressure resistance, low-pressure loss property, good directivity of polymer flow and excellent polymer mixing effect, and it is suitable for use as retainer disposed inside of a filter.