An ion gun system includes an ion gun for irradiating an ion beam; an electric power supply unit for supplying electric power to the ion gun; two mass flow regulators for introducing each of two types of gas in the ion gun; a control unit connected to the electric power supply unit for working as ion gun control means for controlling electric power supplied to the ion gun from the electric power supply unit; and a control unit connected to the mass flow regulators for working as mass flow control means for controlling the flow rate of gas introduced from the mass flow regulators in the ion gun. The control unit as mass flow control means is provided with a function of changing the set value for the flow rate of each of the two types of gas to another set value by changing it stepwise within a range where the ion gun is working stably. Accordingly, shortening of film formation time can be attained.