To provide a cover body installing structure of a liquid vessel attachable to a vessel main body even if a cover body is rotated in either direction.

An installing structure of a cover body 13 to a vessel main body 11 has a plurality of vessel main body side engaging pieces 16a to 16d arranged at intervals on the same circumference F1 in a prescribed position on the outer periphery of the port part 12 of the vessel main body 11 and cover body side engaging pieces 21a to 21d which are arranged at intervals on the same circumference F2 on the inner periphery of the cover body 13 and immovably lock the cover body 13 in the axis 11, 12 direction on the vessel main body 11 by engaging with the lower parts of the vessel main body side engaging pieces 16a to 16d when a rotary position of the cover body 13 to the vessel main body 11 becomes a normal installing position. A locking means is arranged to lock rotation of the cover body 13 to the vessel main body 11.