To provide a mist generating device capable of securely determining leakage in a short time.

The mist generating device includes: a heating medium temperature detection means 81 for detecting the temperature of a heating medium within a heating medium circulation passage 5; a flow control valve 72 provided within the heating medium circulation passage 5; a water supply valve 73 provided in a water supply passage 63 for mist upstream of a liquid-liquid heat exchanger 61; a mist valve 74 provided in the water supply passage 63 for mist downstream of the liquid-liquid heat exchanger 61; a water discharge valve 75 provided in the water discharge passage 64 branched from the water supply passage 63 for mist; and a control part 65 for controlling opening/closing of each valve. The control part 65 performs control to close the flow control valve 72 and the mist valve 74 after completion of mist operation control, discharge hot water within the water supply passage 63 for mist in the liquid-liquid heat exchanger 61 for a predetermined time with the water supply valve 73 and the water discharge valve 75 opened, then, close the water supply valve 73 and the water discharge valve 75 and determines presence/absence of leakage in the liquid-liquid heat exchanger 61 based on decline in the temperature of a heating medium within the heating medium circulation passage 5 detected by the heating medium temperature detection means 81.