ENERGY STORAGE DEVICE

An energy storage device includes a case. The case includes a cover body where an electrolyte solution filling port is formed, and an electrolyte solution plug that closes the electrolyte solution filling port. The electrolyte solution plug includes a shaft part inserted into the electrolyte solution filling port, and a projecting part that projects from a periphery of the shaft part and is bonded to the cover body. In the cover body, a space adjacent to the shaft part is formed around the electrolyte solution filling port, and a tip end of the shaft part is disposed in the electrolyte solution filling port.