PURPOSE: To improve the cooling performance and reduce the weight by providing a cavity part inside at least upper outer circumferential part over the inside of the piston upper circumferential part and top face of a two cycle engine, and sealing sodium (Na) in almost the half of the capacity of this cavity part.

CONSTITUTION: Inside the upper outer circumferential part 2 of the piston of a two cycle engine, a cavity part 3 is arranged. Sodium 4 is pored in the cavity part 3 by the amount corresponding to almost the half of the capacity of the part 3. This Na 4 is applied from a pouring port 5 arranged at one end of the cavity part 3 and then sealed with a cover 6. The sealed Na 4 is liquefied with heat received by the top face of the piston at the low melting point of 93°C. Na 4 is then stirred inside the cavity 3 and heat received by the top face difused toward the skirt of the lower part to lower the temp. at the top face and cancel heat unbalance in the piston 1 itself. It is thus possible to prevent occurrence of trouble such as scuffing of the piston and the like and lighten weight so as to decrease inertia.