To obtain an excellent sealing structure, by providing an uneven part hermetically fitted between the inner and outer peripheral faces of the fitting part of the opening of a medical container and a cap-form member detachably fitted to the opening and forming one side thereof with an elastic member like rubber.

When a protective cap 2 is adapted to the opening 1 of an infusion solution container, the opening 1 is constituted of a plastic intermediate plug 1a, a rubber plug 1b attached to the top face of the intermediate plug 1a, and a plastic outer cylinder 1c with a holder of rubber plug, fitted to the intermediate plug 1a from the upper side of the rubber plug 1b. The opening 1 is airtightly fitted to the cap 2 by the connection of the uneven parts 3, 4. That is, a recess 3 is provided on the outer peripheral face of the outer cylinder 1c and a protrusion 4 is provided at the inner peripheral face of the protective cap 2 respectively. The protrusion part is made of an elastic member 4a. Rubber, a thermoplastic elastomer, etc., are used for the elastic member and simultaneously and integrally formed with the plastic parts side by means of two-color molding method for instance.