The restricting portion 44 is a ring-shaped member partially cut out, and protrudes outward from the cylinder portion 40. The packing 32 is a ring-shaped member made of, for example, synthetic rubber. When the vent plug 18 is attached to the water filling port, the packing 32 ensures the sealing property between the lid 15 and the vent plug 18. The packing 32 is fitted between the head portion 35 and the restricting portion 44. The packing 32 is provided so as to have an outer diameter smaller than that of the head portion 35 and larger than that of the restricting portion 44, and is held at a predetermined position by the restricting portion 44 and the head portion 35.

The through holes 43 allow the inside and the outside of the cylinder portion 40 (cylindrical portion 36) to communicate with each other. Therefore, an opening portion is formed on the inner peripheral surface and the outer peripheral surface of the cylindrical portion 36. The through holes 43 are connected to the exhaust path (not shown) provided in the lid 15. The gas generated in each cell chamber of the lead-acid battery 100 passes from the inside of the cylinder portion 40 to the exhaust path through the through holes 43, and is discharged from the vent hole 24 to the outside. The through holes are provided at two positions facing each other across the central axis L of the cylindrical portion 36.