PURPOSE: To increase a thermal efficiency and raise the temperature of air in a chamber very effectively by a method wherein a heat accumulating space and a heat exchanging part are provided at an evacuating side to prevent the escape of heat and effect the reservation and exchange of the heat effectively.

CONSTITUTION: A reduced pressure friction heat generating mechanism 4 is operated and air in the closed hollow chamer 1 is evacuated at first gradually through an evacuating path 9 by the suction and evacuating effect of a fan 6, then, the pressure in the hollow chamber 1 is reduced and a pressure difference between the inside and the outside of the hollow chamber 1 becomes large gradually. Thereafter, a substantially constant equiliblium condition is maintained when the pressure difference has arrived at a given value. Under this equiblium condition, air stays in a friction heat generating part 8 in the rotating zone of the fan 6 and a frictional effect between the fan 6 is repeated continuously, therefore, the friction heat is generated and the temperature of the air is increased gradually, while the rotating and evacuating effect of the fan 6 tries to send forcibly the heated air into the heat accumulating space 13, therefore, compressing operation is effected, compressin heat is also generated and the temperature of the air in the heat accumulating space 13 is increased suddently by the compression heat as well as the friction heat.