To provide an internal combustion engine by a circular arc-formed piston which generates almost no vibration and noise, has a small friction loss an a good mechanical efficiency, and can be utilized to a reciprocation internal combustion engine which can be composed in a small size and a light weight.

After the reciprocation circular arc movement of a pair of circular arc-formed pistons 1 and 1a is converted into a fluctuation type of rotary movement through a rotor 2 having the left side and the right side parts with the same mass, form, and fluctuation rotary radius, this movement is divided into two parts evenly by two lines of connecting rods 6 and 6a so as to convert into the rotations of two lines of crankshafts 10 and 10a, and the both rotations are outputted.