The invention relates to an apparatus for mixing a gaseous or liquid substance into fiber suspension. The apparatus includes a tubular body, which defines a space that forms a flow channel for the suspension, in which body an inlet and an outlet for the suspension are arranged so that the suspension flows through the flow channel mainly in the axial direction. Further the apparatus comprises a feed member that extends into the flow channel transversely against the flow direction of the suspension and has a cylindrical wall provided with openings for leading the substance from the feed member into the flow channel. At least one protrusion is arranged on the inner surface of the tubular body in the region the feed member. A throttling member is arranged in the flow channel downstream of the feed member in the flow direction of the suspension, and a mixing chamber is formed in the flow channel between the feed member and the throttling member.