PURPOSE: To provide a headbox which is capable of forming a fiber web having a small difference between mechanical characteristics of a transverse direction and a longitudinal direction by improving the dispersibility of the fibers in a pulp suspension.

CONSTITUTION: A first fluid chambered 10 and a second fluid chambered 12 are arranged in the headbox of a papermaking machine and the pulp suspension is supplied to the first fluid chambered 10. The paper web is discharged from the second fluid chambered 12 downstream the first fluid chambered. A turbulence insert 16 is arranged in the upstream substituted 14 of the second fluid chambered 12 and the suspension flows in the spacings between the bottom plate 1, top plate 2 and turbulence insert 16 of the second fluid chambered 12. The front and rear surfaces of the turbulence insert 16 are formed like a step, by which vortex flow is generated in the suspension flowing in the second fluid chambered 12. The turbulence insert 16 is displaceable along the fluid direction, by which the longitudinal cross-section contour of the spacings between the bottom plate 1, the top plate 2 and the turbulence insert 16 is changed with lapse of time and the vortex flow is changed with lapse of time. The fibers in the suspension are thus well dispersed.