To make it possible to increase the life of an accelerating cylinder with small loss at the force for accelerating an airframe.

When a circuit 22 is closed to connect a high voltage large current source 20 to an anode 18 and a cathode 17, a discharge is generated between the both, insulator 14 is evaporated in an evaporating space 13 by the heat of the discharge to generate gas in the space 13 split into forks, an airframe 10 is accelerated by the pressure of the gas and launched from the end of an accelerating cylinder 11. The space 13 is split into the forks to shorten the arms 15, 16 while ensuring the evaporating area which makes it possible to generate the gas of the necessary amount to send the airframe 10, thereby reducing the channel resistance. The pressure necessary to send the airframe 10 is reduced that much to reduce the loss of the force for accelerating the airframe 10.