To provide an ion gun of a penning discharge type capable of narrowing a beam with a low ion beam current at a low acceleration voltage, an ion milling device including the same, and an ion milling method.An ion milling device that controls half width of a beam profile of an ion beam with which a sample is irradiated from an ion gun to be in a range of 200 μm to 350 μm. The device includes: the ion gun that ionizes a gas supplied from the outside, and emits an ion beam; a gas-flow-rate varying unit that varies a flow rate of the gas supplied to the ion gun; and a current measurement unit that measures a current value of the ion beam emitted from the ion gun. The gas-flow-rate varying unit sets a gas flow rate to be higher than a gas flow rate at which the ion beam current has a maximum value based on the current value measured by the current measurement unit and the flow rate of the gas determined by the gas-flow-rate varying unit.