Spark ignition-type internal combustion engine

A spark ignition internal combustion engine, comprising:

• a variable compression ratio mechanism configured to change a mechanical compression ratio;
• a supercharger;
• and
• an ECU configured to control the variable compression ratio mechanism and the supercharger,
• wherein the ECU is configured to carry out a first mechanical compression ratio control and a second mechanical compression ratio control related to the mechanical compression ratio, and boost pressure control to control the boost pressure of the supercharger;
• wherein in said first mechanical compression ratio control, the mechanical compression ratio in a low engine load side operation area is higher than that in a high engine load side operation area to increase an expansion ratio;
• wherein in said second mechanical compression ratio control, the expansion ratio in the low engine load side operation area is lower than that in the low engine load side operation in said first mechanical compression ratio control;
• wherein in the boost pressure control, the boost pressure experiences a step increase when the engine load becomes a set engine load, and
• wherein the ECU is configured to set said set engine load of said boost pressure control higher when said second mechanical compression ratio control is carried out than when said first mechanical compression ratio control is carried out.