bolts 113 is attained and when the separation occurs the cutting force is obtained by a part of the combustion gas flowed into a space 139 between the piston 141 and the partition wall 138 through the second hole 132 b which connects the combustion chamber 135 of the internal combustion pipe 132 with the cylinder portion 142 that is an additional propellant or gunpowder is not required the inflow gas forms a pressure thereby generating a force which causes a relative motion between the piston 141 and the cylinder portion 142 the magnitude of the force is controlled by changing a cross sectional area of the cylinder portion 142 during the combustion of the ejection rocket motor i e the propellant 134 the force pushing the cylinder portion 142 backward is counterbalanced by the rocket thrust and is not sufficient to cut off the shearing bolts 113 for this purpose the piston 141 is in contact with the end of the cylinder portion 142 however the rocket thrust is sharply decreasing at the end of the combustion of the propellant 134 the force balance is broken fig2 is a graph showing a principle of cutting off the shearing bolts and separating the separation device from the missile which represents the force variation with time the force f 1 is a thrust generated by combustion of the ejection rocket motor i.