Filtering device where a fixed guiding structure (11) can receive a sliding element (12) comprising filtering means (21) which come to be placed in succession in the plastic flow. The entry (22) and exit holes (23) of the plastic are greatly reduced in comparison to the diameter of the filtering means (21). Between said holes (22 and 23) and the filtering means (21), an entry cone (26) and an exit cone (27), having an opening angle alpha between 60 and 89 , preferably 80 , are foreseen. The sliding element (12) is made up by two opposite half shells (18, 19) between which the filter holding plate (20) and the filtering means (21) are inserted. The latter can be fitted with a peripheral edging, of lead or other material having a coefficient of thermal expansion higher than the steel's. Between the two half shells a gasket (28) (for instance in steel) supported by a material whose coefficient of thermal expansion is higher than the steel's, can also be foreseen.