To provide an anti-freezing method of phosphoric acid and an anti-freezing device of phosphoric acid, preventing phosphoric acid from freezing under influence of a change of an environmental temperature, by circulating humidified gas to a phosphoric acid fuel cell, and by diluting a phosphoric acid concentration to a target value accurately and quickly.

Humidified gas is circulated to a phosphoric acid fuel cell 10, a phosphoric acid concentration is calculated depending on a flow rate of the humidified gas introduced into the phosphoric acid fuel cell 10, a dew point of the humidified gas introduced into the phosphoric acid fuel cell, and a dew point of the humidified gas exhausted from the phosphoric acid fuel cell, and supplying of the humidified gas is stopped when the calculated value reaches a predetermined value.