To increase the heat generation of a matter to be heated in a container without heating the container by increasing the dielectric constant of the container 1 to be used for heating in a microwave oven, thereby improving the impeadance matching with an incident microwave 6.

The dielectric constant of the container 1 can be increased by dispersing and mixing metallic powder 2, which is not an electromagnetic wave absorber, in the material of the container 1 by a proper ratio and molding the mixture, thereby improving the matching of the incident microwaves 6 with the matter to be heated. Consequently, the reflection of the microwaves 6 on the container 1 is reduced, and microwave intensity in the matter in the container is increased to increase the heat generation of the matter.. Thus, the mert of the microwave oven, which heats the matter directly without heating the container, is made multipliable. Thius, the heating efficiency of the matter is improved, and also a heating period is shortened, energy is saved and the service life of a microwave generator is extended.