Abstract: The present invention improves ashing speed for a treated object and ashing speed uniformity over the treated object by improving microwave transmission efficiency and by generating uniform and strong plasma in a plasma generating chamber. According to the present invention, the microwave treatment apparatus comprises expanding wave guide tubes having an expanded room of a given size on the microwave inlet side of a microwave inlet window 9 which forms a plasma generating chamber, and a partition board with a hole having an aspect ratio other than one in the room. Therewith, a microwave resonant space is formed with interposing the partition board inside the space of the expanded wave guide tube to generate a uniform and strong electric field inside the space of the expanded wave guide tube, and then the microwave is injected into the plasma generating chamber through the microwave inlet window to generate uniform and strong plasma in the plasma generating chamber.