When users use large mobile devices, which are equipped with a large touchscreen, with their single hand, the region distant from their thumb is too distant for users to control. Then, users have to change their hand posture to control the top half, which is a complex action causing an unstable grip. To solve this problem, we present TouchOver, a single-handed stable control technique using a thumb for large mobile devices. This technique solves the problem by transferring touch events on the bottom half of a touchscreen to the upper half. Using TouchOver for the top half and direct touch for the bottom half allows users to control a large mobile device stably, that is a control for all region of a touchscreen only using their thumb without changing the hand posture. We made a widget to make the Android smartphone to detect double tapping its home button, to provide a trigger to activate/deactivate TouchOver. We present the use cases and user studies for measuring the performance of TouchOver.