A steam generator comprising: a water storage chamber which stores water for generating steam in the steam generator, a heating portion which heats water in said water storage chamber so as to perform a steam heating operation, a water supply device which supplies water stored in a water supply tank through a water supply port provided in said water storage chamber and a water supply passage, a control portion which is configured for outputting a supplying- water order signal to said water supply device, and a water discharge passage which discharges water stored in said water storage chamber, wherein said water discharge passage has a first water discharge passage having a U shape to be convex downward, which is in fluid communication with a discharge port provided in said water storage chamber, and a second water discharge passage having an inverted U shape to be convex upward, which is consecutively formed with said first water discharge passage, said steam generator is configured such that water stored in said water storage chamber is discharged through said discharge port and said water discharge passage (25, 35) according to the theory of a syphon, by making said water supply device operate for pushing up a water level of water stored in said water storage chamber to a water discharge passage upmost point which is located upper than a position of a water level stored in said water storage chamber during normal heating process, wherein said steam generator is furthermore configured such that, after completing the steam heating operation, said control portion outputs the supplying- water order signal which supplies water from said water supply tank to said water storage chamber, and discharges water from said water storage chamber according to the theory of a syphon after performing natural cooling for a while until a water temperature in the water storage chamber becomes a predetermined temperature or lower, and at least an inner surface of said first water discharge passage in which scales are likely to be deposited and accumulated structureally, is formed by a nonmetallic material which has a weak bonding force with scales as against metal so as to discharge scales in said first water discharge passage when water in said water storage chamber is discharged, characterized in that at least said first water discharge passage of said water discharge passage is formed by a nonmetallic material which has a weak bonding force with scales as against metal, and said first water discharge passage is formed by a transparent material to allow visual observation of an inside condition of said first water discharge passage. a water storage chamber which stores water for generating steam in the steam generator, a heating portion which heats water in said water storage chamber so as to perform a steam heating operation, a water supply device which supplies water stored in a water supply tank through a water supply port provided in said water storage chamber and a water supply passage, a control portion which is configured for outputting a supplying- water order signal to said water supply device, and a water discharge passage which discharges water stored in said water storage chamber, wherein said water discharge passage has a first water discharge passage having a U shape to be convex downward, which is in fluid communication with a discharge port provided in said water storage chamber, and a second water discharge passage having an inverted U shape to be convex upward, which is consecutively formed with said first water discharge passage, said steam generator is configured such that water stored in said water storage chamber is discharged through said discharge port and said water discharge passage (25, 35) according to the theory of a syphon, by making said water supply device operate for pushing up a water level of water stored in said water storage chamber to a water discharge passage upmost point which is located upper than a position of a water level stored in said water storage chamber during normal heating process, wherein said steam generator is furthermore configured such that, after completing the steam heating operation, said control portion outputs the supplying- water order signal which supplies water from said water supply tank to said water storage chamber, and discharges water from said water storage chamber according to the theory of a syphon after performing natural cooling for a while until a water temperature in the water storage chamber becomes a predetermined temperature or lower, and at least an inner surface of said first water discharge passage in which scales are likely to be deposited and accumulated structureally, is formed by a nonmetallic material which has a weak bonding force with scales as against metal so as to discharge scales in said first water discharge passage when water in said water storage chamber is discharged, characterized in that at least said first water discharge passage of said water discharge passage is formed by a nonmetallic material which has a weak bonding force with scales as against metal, and said first water discharge passage is formed by a transparent material to allow visual observation of an inside condition of said first water discharge passage.