To provide a method for heating a glass substrate uniformly by preventing the temperature of a specified heater block from deviating from a set temperature due to difference of set temperature between adjacent heater blocks.

A plurality of heating chambers 2 sectioned in the carrying direction of a glass substrate 1, and means 3 for carrying the glass substrate 1 intermittently to an adjacent heating chamber are provided. Each heating chamber is provided with a block heater 5 comprising a plurality of heater blocks having an independent control function. A normal automatic control state of the heater block is set in the heating chamber of a firing furnace provided with a power controller 11 for supplying a required heating amount to the heater 5 and a temperature regulator 8. Every time when the glass substrate 1 is carried to the heating chamber, a process for outputting a fixed value forcibly by deviating temporarily from the normal automatic control state and returning back to the normal automatic control state upon elapsing a specified time is repeated.