PURPOSE: To prevent the generation of abnormality due to scaling by controlling the deposition of scales by the circulated boiling device and incorporating a scale remover in the boiling cylinder to scrape off the scales deposited in the boiling cylinder.

CONSTITUTION: A boiling vessel 1 provided with a water feeder 19 and a heater 2, a gas-liq. separator 15 communicating with the vessel 1 and a water discharger 18 are provided. The scale remover 8 having ≥1 protrusion 9 or angular part to approach the inner wall of the boiling cylinder 7 is provided in the cylinder 7. A part of the shaft of the remover 8 is extended to the outside of the cylinder 7 and connected to a power part 12. The power part 12 is actuated to move the remover 8, and the scales deposited in the cylinder 7 are scraped off. Consequently, even it a small amt. of water in the narrow boiling vessel is boiled for a long time, the abnormality due to scaling is not caused.