A headbox apparatus of a paper machine or equivalent has a body (11). Dilution water (arrow L.sub.2) is conducted through at least two rows of ducts (12a.sub.1, 12a.sub.2 . . . ) into each body mixing chambers (13a.sub.1, 13a.sub.2 . . . ). Each mixing chamber (13a.sub.1, 13a.sub.2 . . . ) is limited by vertical walls (T.sub.1), a bottom wall (T.sub.2) and a covering wall (T.sub.3), and a partition wall (16a.sub.1, 16a.sub.2 . . . ). The ducts (14a.sub.1.1, 14a.sub.1.2 . . . ) on the inlet side of the mixing chamber extend into the mixing chamber (13a.sub.1, 13a.sub.2 . . . ) at a distance (h.sub.1) from the ducts (15a.sub.1.1, 15a.sub.1.2 . . . , 15a.sub.2.2 . . . ) on the outlet side of the mixing chamber (13a.sub.1, 13a.sub.2 . . . ), whereby an annular gap (D.sub.1) is left between the aligned duct (14a.sub.1.1, 14a.sub.1.2 . . . 14a.sub.2.1, 14a.sub.2.2 . . . ) on the inlet side and the duct (15a.sub.1.1, 15a.sub.1.2 . . . , 15a.sub.2.1, 15a.sub.2.2 . . . ) on the outlet side. Each mixing chamber (13a.sub.1, 13a.sub.2 . . . ) is filled by dilution water which is conducted through the annular gap (D.sub.1) to join the stock (m.sub.1).
