This invention relates to a drilling device for earth drilling, which comprises a casing part (2) to be placed in a hole to be drilled and a drilling unit (3) in connection with the above during a drilling situation, whereby at a drilling head of the drilling unit there are a drill bit (4) that is to be left after the drilling of the hole at the end of the casing part on the bottom of the hole and a flushing flow arrangement (6) at least for leading of a flushing medium for drilling onto the drill bit's drilling surface (4p), in order to carry out drilling by using the drill bit with a rotational motion (w4), a feeding motion (z4) and/or a hammering motion (t4) and, when the drilling depth increases, by coupling with the drill bit or a hammering device coupled therewith and thereafter with each other successive drill rods in a longitudinal direction (s). In the longitudinal direction (s) behind the drill bit (4) is arranged a separate closing assembly (x) in order to prevent contrary flow through the flushing flow arrangement (6) particularly, when the drilling is interrupted, such as when a drill rod is added or in a corresponding situation, by closing a flow arrangement that connects the flushing flow arrangement (6) with the drill bit's drilling surface (4p).