A restrictor device for controlling a return flow of drilling fluid from a borehole where drilling fluid is provided from a surface installation via a drill string to a bottom-hole assembly is disclosed. Drill pipes forming the drill string are provided with pipe joints including enlarged-diameter pipe portions. An annulus is formed between a riser and the drill string. The annulus is in fluid communication with a drilling-fluid flow path. The restrictor device is arranged in the annulus. The restrictor device includes a restrictor pipe anchored to the riser and surrounds at least one restrictor element placed on an intermediate pipe portion that connects two enlarged-diameter pipe portions. The length of the restrictor device exceeds the largest distance between two consecutive restrictor elements. The restrictor element is a drill-pipe/casing protector with an external diameter larger than a diameter of the pipe joints.