To obtain a waste steam recovering device which can discharge only air to outside from waste steam mixed with the air.

A waste steam passage 1 is connected to a suction port 4 of a steam ejector 2 connected with a drive steam supply pipe 5. A pressure tank 3 is connected to an exit side of the steam ejector 2. An air discharge valve 7 is attached to an upper part of the pressure tank 3, and a steam discharge pipe 8 is connected to a lower part of the pressure tank 3 via a control valve 9. With the air discharge valve 7 and steam discharge pipe 8 connected to the pressure tank 3, only air, which stays in the upper part of the pressure tank 3, is discharged outside via the air discharge valve 7.