The invention provides a process comprising: a crystallization step of concentrating and crystallizing the ammonium salt of sulfuric acid from an aqueous solution that contains the ammonium salt of sulfuric acid; a solid-liquid separation step of solid-liquid separating the ammonium salt of sulfuric acid obtained in the crystallization step, and a crystallization mother liquid recycling step of recycling a crystallization mother liquid obtained in the solid-liquid separation step to at least one step selected from the crystallization step and one or more steps that precede the crystallization step, in which not all of the crystallization mother liquid is recycled.