An internal combustion engine comprises a first raised portion formed on the inner wall of the cylinder head, a second raised portion formed on the top face of the piston at a position opposite to the first raised portion with respect to the axis of the piston, and a third raised portion formed on the inner wall of the cylinder head above the second raised portion. A first flat squish area is formed between the flat peripheral top face of the piston and the flat bottom face of the first raised portion. A second spherical shell shape squish area is formed between the spherical bottom wall of the third raised portion and the spherical rear face of the second raised portion. A surface area ratio of the sum of the first and second squish areas formed on the inner wall of the cylinder head and the first and second squish areas formed on the top face of the piston to the sum of the cross-sectional area of the cylinder bore and the surface area of the inner wall of the cylinder head, except for the surface area of the vertical side wall of the first raised portion, is in the range of 30 percent to 50 percent.