A substrate processing apparatus includes: a cylindrical shaped chamber configured to accommodate a substrate; a movable electrode capable of moving along a central axis of the cylindrical shaped chamber within the cylindrical shaped chamber; a facing electrode facing the movable electrode within the cylindrical shaped chamber; and an expansible/contractible partition wall connecting the movable electrode with an end wall on one side of the cylindrical shaped chamber. A high frequency power is applied to a first space between the movable electrode and the facing electrode, a processing gas is introduced thereto, and the movable electrode is not in contact with a sidewall of the cylindrical shaped chamber, a first dielectric member is provided at the cylindrical shaped chamber's sidewall facing the movable electrode, and an overlap area between the first dielectric member and a side surface of the movable electrode is changed according to movement of the movable electrode.