A variable compression ratio internal engine including at least two block portions connected to each other and moveable relative to each other so that the compression ratio can be varied. The engine includes an input gear for inputting a rotation output from an output gear to the cam shaft. The input gear and a rotation transmission shaft are constituted to be able to slide relative to each other or the output gear and the rotation transmission shaft are constituted to be able to slide relative to each other in the direction of the movement of the one block portion relative to the other block portion such that the meshing between the input gear and the cam shaft side gear or the meshing between the output gear and the crank shaft side gear is maintained when the one block portion is moved relative to the other block portion.