PURPOSE: To completely prevent the leak of air in a tank and to use the compressibility of the air to relax the pulsation of a raw liquid in the tank by sticking the upper end of a seal pipe surrounding an agitation shaft to the under surface of a gland box and submerging the opening part of the lower end thereof in the raw liquid in the closed tank.

CONSTITUTION: An agitating shaft 2 equipment with an agitating blade 2a is hung down in a central shaft part of a cylindrical closed tank 1. A gland packing 5 is inserted into the part of the closed tank 1 through which the agitating shaft 2 is passed. The upper end of a seal pipe 10 surrounding the agitating shaft 2 is stuck to the under surface of a gland box 4 for housing the gland packing 5, and the opening part at the lower end of the seal pipe 10 is submerged in a raw liquid in the closed tank 1. In this way, the through part of the agitating shaft 2 is completely sealed to seal air in the closed tank 1. Since the pulsation of the raw liquid generated by an injection pump transmitted to a pressurizing dehydrator is relaxed, effective dehydration is enabled without broking aggregated flocks in the raw liquid.