In order to consider a next-generation space propulsion system referred to as the “magneto plasma sail,” the magnetic inflation mechanism of a small artificial magnetosphere is investigated. We carry out a two-and-half-dimensional full particle-in-cell simulation, and magnetic inflation mediated by the gyration motion of injected ions is observed. As a result of the gyration motion, an ion-rich region is formed near the direction-reversal position of the injected ions. Magnetic inflation takes place due to the flow of electrons toward the ion-rich region, which carries the field lines of the original magnetosphere. This inflation process is effective for a magnetosphere with a scale comparable to the gyration radius of the injected ions. If the original magnetosphere is much smaller than this, background electrons flow into the ion-rich region outside the magnetosphere, and the inflated magnetosphere is confined to a smaller region. In addition, the thermal effects of background electrons have a similar impact on the inflation process, even if the direction-reversal position is located inside the magnetosphere.