Disclosed is a combustion chamber of an internal combustion engine. A first raised portion having a flat bottom is formed on the inner wall of the cylinder head so that a first flat squish area is formed between the flat bottom of the first raised portion and the flat peripheral portion of the top face of the piston. A second raised portion is 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 has a spherical rear wall so that a second spherical shell shape squish area is formed between the inner wall of the cylinder head and the rear wall of the first raised portion. The first squish flow spouted out from the first squish area moves forward along the top face of the piston. The second squish flow spouted from the second squish area moves forward in the upper interior of the combustion chamber in the direction opposite to the spouting direction of the first squish flow. The first and second squish flows cooperate with each other to create a strong swirl motion rotating about a horizontal axis in the combustion chamber.