A warning device includes a sensing unit, a comparing unit, a controlling unit and a warning unit. The sensing unit sense whether a part of the body of a person is sticking out of the window and generates a sensing signal. The comparing unit detects if the sensing signal is at least a predetermined value. If it is determined that the sensing signal is at least the predetermined value, a part of the body of the person is sticking out of the window and the comparing unit generates a warning signal. The warning unit is used for generating the alarm, and is capable of alternating between an alarm state and standby state. The controlling unit responds to the warning signal to detect whether the warning unit is in the alarm state, and disables the warning unit to start generating the alarm when the warning unit is in the standby state.