Mold temperature control circuit of injection molding device and method for discharging heating medium

In a mold temperature control circuit of an injection molding device, such a molding cycle is repeated that, prior to injection of a resin, a heating-use medium is returned to molds and heated to a temperature suitable for resin injection, after injection of the resin, the heating-use medium is switched to a cooling-use medium, thereby the cooling-use medium is returned to the molds and cooling is conducted to give a temperature equal to or lower than a temperature at which the resin is solidified. In the mold temperature control circuit of the injection molding device, a junction piping is able to commonly use the respective supply pipings of the heating-use medium, the cooling-use medium and the scavenging compressed air to the molds and the respective discharge pipings thereof, and there is provided mold bypass piping coupling the supply-side junction piping of the heating medium with the discharge-side junction piping, a mold bypass on-off valve, a passed heating medium sensor, a mold-temperature adjusting control device which stores a mold temperature determination program and a passed heating medium determination program.