To provide an improved microwave heating device which can attain highly efficient uniform heating.

The electric field of a microwave is made stronger by using a resonant cavity, to make heating efficiency higher. A material to be heated is mounted at a selected place in the resonant cavity where the electric field is weak so that resonance can surely occur in a desired mode without reduction in a Q value even when the material to be heated is mounted in the resonant cavity. The Q value is kept larger than a certain value, to solve the problem of mode selectivity. A signal of a frequency very near a resonance frequency in a TM110 mode is applied to the resonant cavity 1 almost in a shape of a rectangular parallelepiped, and the sheet-like material to be heated is inserted into the electric field of almost uniform strength along a z direction and moved in an x direction. The material to be heated is provided with an almost uniform electromagnetic energy and heated uniformly at a high efficiency.