PURPOSE: To preheat a metallic mold or roll for rolling precisely by a method wherein a pump is installed near a tank connecting it to the tank of a circulating pipe to control the temperature of a heat medium load in the tank under negative pressure of the pump.

CONSTITUTION: Within a circulating pipe 24 connecting a tank 10 to a load 22, a pump 28 is installed near a tank 10 connecting it thereto. The circulating pipe 24 on the suction side of the pump 28 is connected to the tank 10 through the intermediary of a strainer 26 and a stop valve 32. Therefore when the pump 28 is driven, the heat medium oil in the tank 10 may be transferred to the load 22 under suction pressure i.e. negative pressure of the pump 28 while at the same time a heater 12 and cooling water may be controlled through the intermediary of respective controller 44 and 50 by the actions of a temperature senser 16 and a tank thermostat 38 as well as another temperature sensor 42 provided on the load 22 and another load thermostat 40 to maintain the specified temperature of the load 22.