Mixed systems of graphite carbon and polyethylene glycol are found to show an anomalous increase in electrical resistance at certain temperatures. Making use of this property, we have made plane heaters of these graphite carbon‐polyethylene glycol mixed systems. These heaters, when the power was supplied, maintained constant temperature without any temperature control device. It is also found that the constant temperature depends upon the molecular weight of polyethylene glycol used. This makes it possible to obtain a heater which maintains a desired intrinsic constant temperature without any temperature control device.