A piston structure of an engine generates an automatic flow of gas through a hole penetrating from a bowl of the piston toward a lateral side of the piston. The structure provides a reinforcement of the compression and swirling movement in the combustion chamber of an engine, improvement of the mixture function of the fuel and air, reduction of the blowby leaking into the crankcase through the clearance between the cylinder wall and the piston, and prevention of the wear and tear of the cylinder liner and piston by eliminating wet fuel formed on the cylinder wall.