Abstract We present a theoretical model describing magnetar giant flares on the basis of solar flare/coronal mass ejection theory. In our model, preflare activity plays a crucial role in driving evaporating flows and supplying baryonic matter to the magnetosphere. Loaded baryonic matter, which is called “prominence”, is then gradually uplifted via crustal cracking while maintaining a quasi-force-free equilibrium of the magnetosphere. Finally, the prominence is erupted by a magnetic pressure force due to the loss of equilibrium triggered by explosive magnetic reconnection. A giant flare should be induced as a final outcome of prominence eruption accompanied by large-scale field reconfigurations. An essential difference between the solar flare and the magnetar flare is the control process of their evolutionary dynamics. The flaring activity on magnetars is mainly controlled by the radiative process, unlike a solar flare governed by electron conduction. It is full of suggestion that our model accounts for the physical properties of an extraordinary giant flare observed on 2004 December 27 from SGR 1806$-$20, including the source of baryonic matter loaded in the expanding ejecta observed after the giant burst.