Abstract: To recover drilling fluid used in sample drilling at a drilling machine, when the drilling fluid exits the drill hole, and to transport it to a separate cleaning unit wherein solid matter is separated from the drilling fluid. The cleaned drilling fluid is returned to the drilling machine and reused in sample drilling. The cleaning unit has an inlet connection and two or more precipitation basins, where the solid matter accumulates on the bottom of the basin. In the bottom part of at least one precipitation basin there is a valve arrangement for removing drilling fluid sludge containing solid matter from the precipitation basin, which drilling fluid sludge is filtered with a filter arrangement. In the top part of the precipitation basin there is a transfer connection between precipitation basins or an outlet connection for taking cleaned drilling fluid out from the cleaning unit back to the drilling process.