The device (1) is used for feeding an apparatus by gravity simultaneously with at least two streams (ST1, ST2) of solid-state thermoplastic materials (A, B). Said device (1) comprises an inner cooled feeding pipe (10), and an outer feeding pipe (11). At least a lower part (10a) of the said inner cooled feeding pipe (10) is positioned inside the outer feeding pipe (11), with a flowing space (E) between the two pipes (10, 11) for allowing the flow of one stream (ST1) of thermoplastic material (A). The outer feeding pipe (11) comprises an inlet opening (11 b) for the introduction of the first stream (ST1) of thermoplastic material (A) inside the said flowing space (E) and a bottom outlet opening for dropping the first stream (ST1) of thermoplastic material (A) outside the outer pipe, the inner cooled feeding pipe (10) comprises an inlet opening (100a) for the introduction of the other stream (ST2) of thermoplastic material (B) inside the inner cooled pipe (10) and a bottom outlet opening (10c) for dropping the second stream (ST2) of thermoplastic material (B) outside the inner pipe (10).