To provide a highly practical water vapor generating device having a good coefficient of performance with a dual refrigerator using little toxic refrigerant.

The water vapor generating device comprises a lower side refrigerant circuit consisting of a refrigerant outward pipe 10 having one end connected to the discharge side of a lower side compressor 9 and the other end connected to a primary side flow path inlet 7c of a cascade condenser 7 and a refrigerant homeward pipe 11 having one end connected to an outlet 7d and the other end connected to the suction side of the lower side compressor 9 via a pressure reducing valve 13 and an evaporator 14, and an upper side refrigerant circuit 1 consisting of a refrigerant outward pipe 4 having one end connected to the discharge side of an upper side compressor 3 and the other end connected to a secondary side flow path inlet 7a of the cascade condenser 7 via a condenser 5 and a pressure reducing valve 6 and a refrigerant homeward pipe 8 having one end connected to an outlet 7b and the other end connected to the suction side of the upper side compressor 3. Refrigerant in the upper side refrigerant circuit is water, and refrigerant in the lower side refrigerant circuit is octafluorocyclobutane.