An apparatus for controlling medium flow, especially for controlling dilution liquid flow in connection with the headbox of a paper-making machine or a cardboard-manufacturing machine, which apparatus comprises a valve which controls dilution flow, which valve consists of a valve housing, a stem part and a flow channel between an inlet and an outlet of the valve. The stem part is movable in the housing between at least two positions, a first position in which the flow path between the inlet and the outlet is open, and a second position in which the flow path is closed, and the stem part and/or housing is designed to direct the medium flow at least in the vicinity of the outlet opening.