PURPOSE: To provide the cylinder head of a flat head combustion chamber type two cycle engine, in which the improvement of scavenging efficiency can be compatible with the enlargement of a compression ratio in a combustion chamber.

CONSTITUTION: An intake port 2, an exhaust port 3, and a sub-combustion chamber 4 are formed in a cylinder head of a flat head combustion chamber type overhead valve engine, and an air intake outlet of the intake port 2 and an exhaust inlet 31 of the exhaust port 3 are opened in the undersurface 1A of the cylinder head 1 facing a main combustion chamber 10 through a valve-spaced part 5. Steps are provided on the undersurface 1A of the cylinder head to open-form an intake circulating frontage 25 and an exhaust circulating frontage 35. On the inner end faces of the respective circulating frontages 25, 35, an intake valve seat-mouthpiece 22 and an exhaust-valve seat-mouthpiece 32 are provided flush with each other. The effective cross-sectional area of each of the circulating frontages 25, 35 is formed to be narrower on, the side of the valve-spaced part 5, and the effective cross-sectional area of each of the respective circulating frontages 25, 35 is formed to be wider away from the valve- spaced part 5.