A device is described for maintaining a predominately constant pressure on, and flow of drilling fluid in, a drill string (100) where drilling fluid is supplied via a circulation system for drilling fluid. The device comprises a mainly elongated, internally hollow body (10) arranged to surround the drill string where the hollow body (10) comprises at least an upper pressure chamber (40a) and a lower pressure chamber (40b) connected with respective inlet and/or outlet (50a, 50b) for drilling fluid from or to the drilling fluid circulation system, as said pressure chambers (40a, 40b) are able to be closed and separated by an intermediate valve (70) arranged for circulation of drilling fluid into or out of the drill string (100) during coupling up or disconnecting of a new length of drill string (100). A method to maintain the mainly constant pressure of drilling fluid in a drill string is also described.