An automatic power cut-off device for emergency situations which is capable of immediately sensing occurrence of an emergency situation such as an accident of a vehicle striking against an obstacle during its running at a high speed, and automatically cutting off electric power flowing through the vehicle, thereby protecting a driver or other passengers in the vehicle from fire or explosion. The automatic power cut-off device includes an impact sensing device having an impact sensor adapted to sense an impact, thereby generating an impact sensing signal, and a solenoid driving circuit for activating a solenoid in response to the impact sensing signal. A return knob is also provided which rotates between a first position, in which connecting terminals respectively coupled to an electric power source and a load are electrically connected to each other, and a second position, in which the connecting terminals are disconnected from each other. The return knob is maintained in its first position in an inactive state of the solenoid as it is engaged with an engagement extension provided at the solenoid. When the solenoid is activated, the return knob is disengaged from the engagement extension, so that it rotates to its second position by a tension coil spring. Thus, the supply of electric power is cut off.