PURPOSE: To suppress the thermal expansion of a rotary drum further to reduce its rotary resistance, by interposing a low friction material between a cooling layer and a bottom surface of the rotary drum.

CONSTITUTION: Since a rotary drum 1 rotates while its bottom surface is brought into contact with the upper surface of a low friction plate 2, friction heat is generated between these members 1, 2. However, the rotary drum 1 is cooled immediately transmitting the friction heat, generated between the bottom surface of the rotary drum 1 and the low friction plate 2, to cooling fluid in a cooling chamber 32 because the cooling fluid the cooling chamber 32 is brought into contact with the bottom surface of the low friction plate 2. The rotary drum 1, which drains the cooling fluid from a drain path 34 while continuously supplies the new cooling fluid from a fluid filling path 33, prevents a cooling function from worsening. Consequently, the low friction plate 2 and a cooling tank 3 including the rotary drum 1 are prevented from generating thermal expansion due to the friction heat.