A stool sampling device comprising a reagent vessel, an intermediate piece and a sample lance, said reagent vessel having a top access opening and at least one flange on its outer wall; said intermediate piece having a cylindrical top opening, at least one integrally formed fitting guide which cooperates with the at least one flange of the reagent vessel, and a bottom plug-in cone, said intermediate piece having means for close-fitting with the inner wall of the reagent vessel at the access opening, and a bottom fitting hole connecting with the interior of the reagent vessel; and said sample lance having a head part and a lance rod for taking a stool sample, wherein introducing the lance rod into the intermediate piece through the fitting hole results in the closure of the reagent vessel by the head part; characterized in that the intermediate piece has a top cylindrical inner wall with at least one sealing bead; the head part of the sample lance has at least one bottom taper sealing ring; the at least one taper sealing ring in the head part of the sample lance cooperates with the least one sealing bead in the intermediate piece, audibly engaging in a snap connection when the lance rod is fully introduced through the fitting hole of the intermediate piece, thereby creating radial tension between the outer wall of the head part and the inner wall of the intermediate piece and radially sealing the access opening of the reagent vessel; at least one fitting recess and at least one fitting window are formed on the cylindrical wall (52, 54) of the intermediate piece, and at least one long flange and at least one short ramped flange are integrally formed on the outer wall of the head part; wherein the at least one fitting recess is arranged to receive the at least one long flange, so that the head part is guided into the intermediate piece, thereby allowing the cooperation of the at least one fitting window of the intermediate piece with the at least one short ramped flange and irreversibly restraining the movement of the head part upon engagement with the intermediate piece and closing the reagent vessel. said reagent vessel having a top access opening and at least one flange on its outer wall; said intermediate piece having a cylindrical top opening, at least one integrally formed fitting guide which cooperates with the at least one flange of the reagent vessel, and a bottom plug-in cone, said intermediate piece having means for close-fitting with the inner wall of the reagent vessel at the access opening, and a bottom fitting hole connecting with the interior of the reagent vessel; and said sample lance having a head part and a lance rod for taking a stool sample, wherein introducing the lance rod into the intermediate piece through the fitting hole results in the closure of the reagent vessel by the head part; characterized in that the intermediate piece has a top cylindrical inner wall with at least one sealing bead; the head part of the sample lance has at least one bottom taper sealing ring; the at least one taper sealing ring in the head part of the sample lance cooperates with the least one sealing bead in the intermediate piece, audibly engaging in a snap connection when the lance rod is fully introduced through the fitting hole of the intermediate piece, thereby creating radial tension between the outer wall of the head part and the inner wall of the intermediate piece and radially sealing the access opening of the reagent vessel; at least one fitting recess and at least one fitting window are formed on the cylindrical wall (52, 54) of the intermediate piece, and at least one long flange and at least one short ramped flange are integrally formed on the outer wall of the head part; wherein the at least one fitting recess is arranged to receive the at least one long flange, so that the head part is guided into the intermediate piece, thereby allowing the cooperation of the at least one fitting window of the intermediate piece with the at least one short ramped flange and irreversibly restraining the movement of the head part upon engagement with the intermediate piece and closing the reagent vessel.