PURPOSE: To obtain a steam supply structure which enables the establishment of security for full-water boiling in an auxiliary boiler and of security of boiling easily and at a lower expense.

CONSTITUTION: A main boiler 6 is provided having a steam reservoir 14 to store steam to be generated, an auxiliary boiler 60 for auxiliary use to the main boiler 6 and the auxiliary boiler 60 is provided with an auxiliary boiler steam supply path 6 to introduce steam to be generated by the auxiliary boiler 60 to the steam reservoir 14 and a safety valve mechanism 62 to build a steam supply structure. A first valve mechanism 63 is provided in the auxiliary boiler steam supply path 61 to switch a passage between the communication position and the interruption position. A bypassing path 64 is provided to connect an auxiliary boiler steam supply path part 61a on the side of the auxiliary boiler from the first valve mechanism 63 and a releasing path part 62d on the downstream side from a safety valve 62a provided in the safety valve mechanism 62 and a second valve mechanism 65 is provided in the bypassing path 64 to switch the passage between the communication position and the interruption position.