The invention relates to a method and an apparatus for treating high-consistency fiber suspension. The method and apparatus according to the invention are especially suitable for short distance conveyances of thick fiber suspensions (consistency more than 15 %) in pulp and paper industry, for example for discharge of mass towers either with a pump or without any actual pump. The treatment of fiber suspension with a consistency more than 15 % is not possible with the known technique without a displacement type of pump, which is expensive and easy to break. Additionally in discharging of the mass tower(4), drop leg, etc. fiber suspension causes trouble by arching in the container in such a way that it forms an open chamber around the pump located at the bottom of the container which chamber is slowly filled by fiber suspension. Said problems are solved or minimized by arranging a feed apparatus (31) in the pulp chamber, which feeds fiber suspen­sion to a fluidizing rotor (21), which fluidizes the fiber suspension, whereafter the suspension flows onwards. On the other hand, the feeder apparatus (3) is characterized in that it is to raise the pressure of the fiber suspension sufficiently for the fluidization, but not too high, in which case the operational members would be stressed redundantly. For said reason fiber suspension is fed excessively to the rotor (21), whereby the feeding pressure of fiber suspension is controlled by throttling devices (34) which are arranged separate from the feeder apparatus (3) in the back-circulation duct of passage.