PROBLEM TO BE SOLVED: To provide a method for operating a gas production system using a circulating fluidized bed gasification furnace, in which the start-up time of the gas production system is shortened while avoiding accumulation of a fluid medium at an unexpected place.SOLUTION: When an operation of the gas production system is started, firstly, air is supplied to an inlet loop seal, the gasification furnace, and an outlet loop seal to form a fluidized bed of the fluid medium (S312). The circulation of the fluid medium is started, a combustion furnace is driven and the heating of the fluid medium is started (S314). When the temperature of the fluid medium becomes equal to or higher than the temperature when steam is not condensed (YES at S316), the supply of air to the outlet loop seal is stopped and the circulation of the fluid medium is stopped (S318). Subsequently, the fluid gas to be supplied to the inlet loop seal and the gasification furnace is changed over from air to steam (S320). When the change
