To provide a microwave oven including an induction structure of electromagnetic waves for dispersing the electromagnetic waves generated from a magnetron with a maximum electric field intensity in the electronic range.

Electromagnetic waves generated from a magnetron of a microwave oven are dispersed into a cooking chamber 160 from an opening 130 througha waveguide 120. A plurality of annular electric field staircases 220, 230 are formed on an upper side wall in the cooking chamber 160 for sustaining maximum spherical electric field intensity. Each annular electric field staircase on the upper side wall is constructed such that a large radius annular staircase is lower in its height from a bottom surface than a small radius one and that a center part of the upper side wall where the opening is formed becomes highest. A plurality of annular electric field staircases symmetrical to the foregoing plurality of the electric field staircases on the upper side wall are formed also on the bottom surface of the cooking chamber. Also in a waveguide 120 electric field staircases are formed such that the electromagnetic waves sustain a maximum electric field intensity. Hereby, ultrahigh frequency waves are uniformly dispersed in the cooking chamber keeping a maximum electric field intensity to improve cooking efficiency.