PURPOSE: To permit quick change of a production kind currently under production to a production kind which is different in gob temp. by executing the change of the set temp. in the respective zones in a force hearth and the temp. in a spout by a specific method at the time of changing the kind of product.

CONSTITUTION: Prior to the start of a product change operation, the glass temp. in the force hearth is successively changed to the temp. required for the product after the change from the zones nearer the furnace while the glass temp. in the spout is maintained at the temp. requested for the kind of product currently under production. The change of the glass temp. in the spout is quickly completed after the product change operation is started.