In an electronic device capable of being driven on a battery, there is provided a docking system configuration in which a docking unit mainly serving as an extension can be attached/detached to/from a master unit by using a system bus as a bridge. The docking unit is adapted to operate alone upon start-up of a power source even while being detached from the master unit. In each of the master unit and the docking unit, detection is made for whether the units are docked each other and for whether a residual capacity of a driving battery in steps is decreased. When it is detected that the residual capacity of the driving battery is decreased, either of operation suspending processing or operation end processing is selectively executed.