A tool unclamping device is a device capable of unclamping a tool clamped to a spindle of a machine tool, and comprises a casing, an output shaft set on this casing and capable of outputting power necessary for unclamping the tool, a motor serving as a power source capable of supplying power for driving the output shaft, a power transmission mechanism transmitting the power from the motor to the output shaft and a gas cylinder serving as an auxiliary power source capable of supplying auxiliary power to this power transmission mechanism.