Abstract: In an apparatus for treatment of an ammoniacal waste liquid comprising a reaction tank for effecting a decomposition reaction of fixed ammonia by adding a calcium series alkali so as to obtain a reaction product comprising free ammonia and a calcium salt, a solid-liquid separator for effecting separation of the calcium salt so as to obtain a separated liquid, a distillation column for steam-stripping of the free ammonia from the separated liquid, and a pipe passage connecting consecutively said reaction tank, said separator and said distillation column, the improvement comprising a heat aging tank interposed in the pipe passage between the solid-liquid separator and the distillation column so that the calcium salt present in a dissolved state in the separated liquid may be separated out and aged in the aging tank where the liquid is raised to the temperature at the top of the distillation column, thus preventing blocking in the distillation column.