A bayonet connection structure between a container mouth portion and a cap is disclosed. The cap includes multiple slip prevention claws spaced apart from each other. One of the slip prevention claws is elastically displaceable in a radial direction. A rotation prevention projection is provided between the elastically-displaceable slip prevention claw and the other slip prevention claws. The mouth portion has a hollow cylindrical portion with an outer circumferential surface provided with multiple slip prevention claw (SPC) engagement portions for the slip prevention claws. The SPC engagement portions are as many as the slip prevention claws. Each of the SPC engagement portions has an end in a clockwise direction provided with a rotation prevention stopper. The mouth portion includes a rotation prevention rib to be got over by the rotation prevention projection of the cap when the cap is rotated.