Abstract: Self-blocking fishing device to be used in petroliferous well drilling tools, wherein said tools are preferably Positive Displacement Downhole Drilling Motors, Drilling mud motors, and the similar. The device includes a substantially cylindrical and hollow rod, and a sliding piece which includes an orifice for said rod to pass through, wherein the rod includes an end screwed portion for connecting the tool, whereas in a distal portion from said screwed end, said rod includes a reducing diameter zone, presenting the sliding piece an internal diameter slightly larger than the outer diameter of said rod, wherein in an lower portion of said sliding piece the inner diameter increases, including said widening at least one liquid flowing opening, being said rod and said sliding piece mutually fixed by a transversal bolt.