PURPOSE: To protect the interior of a pump chamber from being contaminated by lubricant or abrasive powder of sealing material, in a Root's pump, by providing a hydro-magnetic seal at a section where the rotor shaft penetrates through the barrier wall of the pump chamber.

CONSTITUTION: Hydro-magnetic seals 19W22 are arranged between shafts 9, 10 and barrier walls 4, 5 at the side of rotors 1, 2 where the shafts 9, 10 penetrate through the barrier walls 4, 5. Since rotary boards 17, 18 fixed to the shaft 10 rotate together with the shaft 10, lubricant in oil tanks 7, 8 is dispersed to lubricate gears 15, 16 and bearings 11W14. The lubricant passes through the bearing sections and reaches to respective sections of the seals 19W22 but does not reach to the pump chamber 6 side.