An ammonia-controlling method for fermentation, which is for controlling ammonia concentration of a culture medium contained in a culture tank, wherein ammonia concentration in the culture tank is controlled by using an ammonia-controlling apparatus comprising at least an ammonia feeder that supplies ammonia to the culture tank, an ammonia sensor that responds to non-ionized ammonia in the culture medium contained in the culture tank, and a control part connected to the ammonia feeder and the ammonia sensor, and which method comprises the following steps performed by the control part: a non-ionized ammonia concentration calculation step of calculating non-ionized ammonia concentration of the culture medium by substituting the signal from the ammonia sensor into a calibration curve representing relation between non-ionized ammonia concentration of the culture medium and the signal from the ammonia sensor, and an ammonia supply direction step of directing the ammonia feeder to supply ammonia to the culture tank when the calculated non-ionized ammonia concentration is lower than a predetermined concentration, wherein the ammonia-controlling apparatus is further connected to a pH sensor for measuring pH value of the culture medium contained in the culture tank, which method further comprises the following step performed by the control part: a total ammonia concentration calculation step of calculating total ammonia concentration from the non-ionized ammonia concentration calculated in the non-ionized ammonia concentration calculation step and a pH value measured with the pH sensor on the basis of an ammonia dissociation curve representing an existing ratio of non-ionized ammonia concentration and ammonium ion concentration in the culture medium contained in the culture tank at each pH value, and wherein, in the ammonia supply direction step, the total ammonia concentration is used instead of the non-ionized ammonia concentration. a non-ionized ammonia concentration calculation step of calculating non-ionized ammonia concentration of the culture medium by substituting the signal from the ammonia sensor into a calibration curve representing relation between non-ionized ammonia concentration of the culture medium and the signal from the ammonia sensor, and an ammonia supply direction step of directing the ammonia feeder to supply ammonia to the culture tank when the calculated non-ionized ammonia concentration is lower than a predetermined concentration, wherein the ammonia-controlling apparatus is further connected to a pH sensor for measuring pH value of the culture medium contained in the culture tank, which method further comprises the following step performed by the control part: a total ammonia concentration calculation step of calculating total ammonia concentration from the non-ionized ammonia concentration calculated in the non-ionized ammonia concentration calculation step and a pH value measured with the pH sensor on the basis of an ammonia dissociation curve representing an existing ratio of non-ionized ammonia concentration and ammonium ion concentration in the culture medium contained in the culture tank at each pH value, and wherein, in the ammonia supply direction step, the total ammonia concentration is used instead of the non-ionized ammonia concentration. a total ammonia concentration calculation step of calculating total ammonia concentration from the non-ionized ammonia concentration calculated in the non-ionized ammonia concentration calculation step and a pH value measured with the pH sensor on the basis of an ammonia dissociation curve representing an existing ratio of non-ionized ammonia concentration and ammonium ion concentration in the culture medium contained in the culture tank at each pH value, and wherein, in the ammonia supply direction step, the total ammonia concentration is used instead of the non-ionized ammonia concentration.