PURPOSE: To ensure safe and reliable operation of a heat accumulating type air conditioner using a heat accumulating medium capable of accumulating cold and to increase its heat exchanging efficiency, by forming a passage by-passing a heat exchanger used for cooling a room to be air conditioned, in the manner that it can be connected or disconnected selectively.

CONSTITUTION: In accumulating cold, a by-pass passage 9 is connected in the circuit by opening a gate valve 10, while a heat exchanger 7 used for cooling a room to be air-conditioned is disconnected by closing a gate valve 11. Resultantly, cooling water 8 in a heat accumulating or regenerative tank 4 is cooled by a cooling heat exchanger 3 while it is circulated through the by-pass passage 9 by a circulating pump 6, so that cold is accumulated in the tank 4. In space- cooling operation, on the other hand, the valve 10 is closed and the valve 11 is opened for feeding cold in the tank 4 to the heat exchanger 7. Thus, it is enabled to ensure safe and reliable operation of the air conditioner and to increase its heat exchanging efficiency.