PURPOSE: To enable a high compression ratio to be obtained while securing a required squish zone by integrally superposing on a semispherically projected part which has been formed on the top of a piston, another semispherically projected part having a smaller radius of curvature than that of the former.

CONSTITUTION: In the top of a piston 1, on a semispherically projected part 1a of a radius R1 of curvature, a semispherically projected part 1b having a radius R2 of curvature smaller than the radius R1 of curvature is integrally superposed. Accordingly, two semispherically projected parts 1a and 1b are stepwisely formed. Thereby a required squish zone Z can be secured between the semispherically projected part 1a and a cylinder head 13, and the presence of the semispherically projected part 1b reduces the volume of the combustion chamber S formed between the piston 1 and the cylinder head 13, thus the compression ratio of mixture can be increased.