A cycle is repeatedly carried out, the cycle including: a material feeding step of feeding a material whose amount is larger than an amount that is needed to form a single mold; a mixing step; an ejection step; and a measurement step of measuring, once before or after the ejection step, an amount of foamed mixed sand contained in a mixing tank. The material at least partially is not fed in the material feeding step in the cycle in which the amount of the foamed mixed sand is not less than a first threshold.