To let negative ions act on a heat insulating process of cooked rice, to suppress propagation of bacteria contained in the cooked rice, to set the heat insulating temperature low, and to insulate heat safely with less heat insulating degradation by providing a means generating the negative ions for preventing the propagation of the bacteria in the cooked rice.

A negative ion generation means 1 receives corona discharge from an electrode of a high-voltage generation means 2 impressed to a high voltage so as to generate negative ions. Then, a heating part control means 6 controls the heating quantity to the heaving part 5 for heating a cooking jar and rice 8 based on a rice quantity by a weight sensor measuring means and the cooked rice temperature by temperature sensor measuring means 12 and at the same time controls the high-voltage generation means 2 via an energization control means so as to make the negative ion generation quantity of the negative ion generation means 1 to the decided quantity.