Method to maintain a predominately constant pressure on, and flow of drilling fluid in, a drill string during drilling, where drilling fluid is supplied via a circulation system for drilling fluid, comprising the following steps: to arrange a mainly elongated, internally hollow body around the drill string where said body comprises a pressure chamber with an upper and a lower pressure chamber (40a, 40b) separated by an intermediate valve, to separate the drill string so that the pipe coupling, which is separated from the drill string, is inside the hollow body and that an upper and a lower part of the drill string is locked and sealed in the respective upper and lower chambers (40a, 40b) at the same time as drilling fluid is supplied to the inside of the body, such that drilling fluid at a pressure corresponding to the pressure in the drill string is supplied to the body, the intermediate valve is closed, which separates the upper and lower pressure chambers (40a, 40b), to maintain pressure in the lower chamber and to equalise the pressure in the upper chamber to the surroundings, to release and pull up the upper part of the drill string to collect a new length of drill string with a number of drill pipes, to lead the new length of drill string into the upper chamber whereupon it is locked and sealed in the upper chamber, to supply drilling fluid to the upper chamber to obtain a pressure corresponding to the pressure in the lower chamber, whereupon the intermediate valve between the upper and the lower chambers (40a, 40b) is opened, and to couple together the upper and the lower part (120, 130) of the drill string whereupon the drill string is released and the pressure is released in the body, characterised in that, prior to the step of separating the drill string, said body is lifted up along the drill string so that the pipe coupling is made accessible when there is a need to insert a new length of drill string, and to drive in a roughneck and, at least, break up the coupling, whereupon the roughneck is driven away from the drill string and the body is lowered down over the coupling which now has a soft-break status still with drilling fluid under pressure and circulating inside. to arrange a mainly elongated, internally hollow body around the drill string where said body comprises a pressure chamber with an upper and a lower pressure chamber (40a, 40b) separated by an intermediate valve, to separate the drill string so that the pipe coupling, which is separated from the drill string, is inside the hollow body and that an upper and a lower part of the drill string is locked and sealed in the respective upper and lower chambers (40a, 40b) at the same time as drilling fluid is supplied to the inside of the body, such that drilling fluid at a pressure corresponding to the pressure in the drill string is supplied to the body, the intermediate valve is closed, which separates the upper and lower pressure chambers (40a, 40b), to maintain pressure in the lower chamber and to equalise the pressure in the upper chamber to the surroundings, to release and pull up the upper part of the drill string to collect a new length of drill string with a number of drill pipes, to lead the new length of drill string into the upper chamber whereupon it is locked and sealed in the upper chamber, to supply drilling fluid to the upper chamber to obtain a pressure corresponding to the pressure in the lower chamber, whereupon the intermediate valve between the upper and the lower chambers (40a, 40b) is opened, and to couple together the upper and the lower part (120, 130) of the drill string whereupon the drill string is released and the pressure is released in the body, characterised in that, prior to the step of separating the drill string, said body is lifted up along the drill string so that the pipe coupling is made accessible when there is a need to insert a new length of drill string, and to drive in a roughneck and, at least, break up the coupling, whereupon the roughneck is driven away from the drill string and the body is lowered down over the coupling which now has a soft-break status still with drilling fluid under pressure and circulating inside.