To provide a plasma treatment device and a method generating plasma surely in a short time and stably maintaining the same until it transits to a matching position after it has once been generated.

The plasma treatment device is provided having a vacuum vessel in which a treated object is housed and is put under a plasma treatment in vacuum or under reduced pressure, an impedance matching unit arranged between a microwave generating source for generating microwave and the vacuum vessel for making impedance adjustment, and a controlling part controlling operation of the impedance matching unit in accordance with a matching state of the impedance matching unit, an intensity distribution of microwaves necessary for plasma generation in the matching state, and a matching state of the impedance unit where a reflective wave of the microwave becomes the lowest during the plasma treatment.