To facilitate the manufacture while preventing from affecting a breather valve the torsion and deformation in a packing part which are generated when tightening a cap.

The breather valve seal is equipped with a packing part 2 sandwiched between a port of a tank and a cap for sealing the port so as to seal the clearance therebetween, a breather valve part 3 for automatically adjusting pressure fluctuations in the tank, a coupling part 4 for coupling the packing part 2 and the breather valve part 3, and a low rigidity region 5 in which the packing part 2, breather valve part 3 and coupling part 4 are integrally molded by a rubber material and which is deformable more easily than the periphery. The low rigidity region 5 is preferable to be a thin-walled part 7 formed by an annular groove 6 surrounding the breather valve part 3. It is also preferable to include a high rigidity region 9 having higher rigidity than the coupling part 4 in a base part 8 of the breather valve part 3.