Provided is a novel compression ratio adjustment device for an internal combustion engine, with which knocking-resistant performance can be improved and increases in the temperature of exhaust gas canbe minimized. This compression ratio adjustment device is configured to make the mechanical compression ratio relatively small and make the mechanical expansion ratio at the same time relatively large in regions of large load on the internal combustion engine. With this configuration, because control is performed to make the mechanical compression ratio smaller and the mechanical expansion ratioat the same time larger in regions of large load on the internal combustion engine, knocking-resistant performance can be improved, increases in the temperature of exhaust gas can be minimized, and heat damage to exhaust system components can be reduced.