A method for controlling in-plane uniformity of a substrate processed by plasma-assisted process in a reactor, includes: supplying a principal gas to a reaction space, and discharging radially the principal gas from the reaction space through an annular duct; and supplying an secondary gas to the reaction space from an area in close proximity to an outer periphery of a susceptor, outside an outer circumference of the substrate as viewed from above, so as to flow at least partially in an inward direction passing the outer circumference of the substrate, reversing the direction of the secondary gas to flow toward the annular duct in a vicinity of the outer circumference of the substrate, and discharging radially the secondary gas together with the principal gas from the reaction space through the annular duct.