PURPOSE: To reconcile the reduction of manufacture cost of a cylinder block and the improvement of engine output by enabling the mating faces of clank case and the cylinder block to be easily processed into planes without scarifying the freedom in design of a lead valve housing, an exhaust port housing, etc.

CONSTITUTION: In a two-cycle engine 1A, where the mating faces B of a crank case 2A and a cylinder block 3A are made into planes crossing the center axis O of a cylinder bore 6 made inside the cylinder block 3A and an exhaust port 19 leading to outside from the cylinder bore 6 is provided, the mating faces B cross an exhaust port 23 where an exhaust port 19 is made.