To rapidly raise the surface temperature of a warming roll to a predetermined temperature when a rubber base material to be warmed is replaced.

An external heater 3 is moved to an operation position shown by Fig. (b) from a standby position shown by Fig. (a) simultaneously with the completion of the warming work of the rubber base material used in the present production to start the heating of the outer peripheral surface of the warming roll 1. The temperature of the outer peripheral surface of the warming roll 1 is measured by a temperature sensor 10 and, when it becomes a predetermined temperature or above, the external heater 3 is moved to the standby position as shown by Fig. (c) to start the warming of the rubber base material.