A method for producing fluorine gas including electrolyzing an electrolyte in an electrolytic cell, measuring the average particle size of a mist contained in a fluid generated in the inside of the electrolytic cell in the electrolyzing the electrolyte, and sending the fluid from the inside to the outside of the electrolytic cell through a flow path. The flow path in which the fluid flows is switched in accordance with the average particle size of the mist measured in the measuring the average particle size, such that the fluid is sent to a first flow path when the average particle size of the mist measured in the measuring the average particle size is not more than a predetermined reference value, or the fluid is sent to a second flow path when the average particle size is more than the predetermined reference value.