with the rotation of the worm wheel gear 60 referring to fig2 a variable compression ratio device of the present invention may be applied to a four cylinder internal combustion engine one would appreciate that a variable compression ratio device of the present invention can be applied to other internal combustion engines a main piston 70 is provided in each cylinder and the volume of a main combustion chamber 10 a is varied in accordance with the up and down movement of the main piston 70 a sub combustion chamber 20 is provided in the cylinder head 10 so as to communicate with the main combustion chamber 10 a of each cylinder and the sub piston 30 is movably installed in each sub combustion chamber 20 so as to vary the volume of the sub combustion chamber 20 one end of the connecting rod 40 is integrally connected to the sub piston 30 and the eccentric cam 50 is coupled for example by press fitted to the other end of the connecting rod 40 each eccentric cam 50 is integrally and rotatably connected to a rotary shaft 50 a and the rotary shaft 50 a is connected to a driving motor 64 through the worm wheel gear 60 and the worm gear 62 thus the rotary shaft 50 a rotates each eccentric cam 50 upon receipt of rotary force of the driving motor 64 thereby varying the compression ratio of each cylinder one.