An optical monitor device of the present microwave plasma etching device has: a monitor head located in a position more radially inward than the edge of a semiconductor wafer W mounted on a susceptor, more radially outward than a coaxial pipe, and above a cover plate; an optical waveguide for monitoring provided vertically below the monitor head, and longitudinally traversing the cooling plate, a dielectric plate, and a dielectric window; and a monitor main body optically connected to the monitor head via an optical fiber.