Considering the case where the distance h₂ is greater than or equal to h₁+α, the distance in the axis L direction from the upper end portion 62b of the first splash-proof plate 62 to the end portion 45a of the slit 45 increases. Therefore, as compared with the case where the distance h₂ is less than h₁+α, the electrolyte solution that has entered the inside of the cylindrical body 36 due to vibration or the like easily hits the inner wall portion of the cylindrical body 36, and the electrolyte solution that has entered the inside of the cylindrical body 36 is less likely to be discharged to the outside of the cylindrical body 36. Also in this case, as in the case described above, when vibration or the like is further applied to the electrolyte solution that has entered the cylindrical body 36, the electrolyte solution may flow up in the cylindrical body 36 and overflow.

As described above, since the vent plug 18 satisfies the above relational expression (1), the vent plug 18 is less likely to cause overflow due to vibration or the like (excellent in dynamic overflow performance).

In the vent plug 18, the upper end portion 62b of the first splash-proof plate 62 is located closer to the first end portion 50 (on the upper side) than the start end portion 42a of the screw thread portion 42.