Provided is a method of manufacturing a weight-saved foamed gypsum board having a gypsum core with high strength and excellent adhesion properties of the gypsum core with base papers for gypsum board, which method is capable of adjusting the sizes of pores in a gypsum core of a weight-saved gypsum board product into a desired range in a short time and at a low cost, even if the kinds and the compounding ratios of gypsum raw materials vary. The method of manufacturing a weight-saved gypsum board in which pores with a predetermined size are distributed in a gypsum core, includes the steps of blowing air into a foaming agent to produce foams, mixing the foams into a kneaded material that contains calcined gypsum and water to obtain foamed gypsum slurry, pouring the foamed gypsum slurry into a space between upper and lower base papers for gypsum board, shaping the base papers and the foamed gypsum slurry into a board shape, roughly cutting off and subsequently drying the board-shaped one, and cutting off the dried and shaped one into a product dimension, wherein the method further comprises the step of preliminarily adding a pore size adjusting agent for adjusting sizes of foams distributed in the foamed gypsum slurry to one of a stock solution of the foaming agent and a mixture of a stock solution of the foaming agent and water to obtain the foaming agent for producing foams with desired sizes.