Disclosed is a fluid container having a flexible bag containing a diluent and having a closing film at its upper end, a capsule connected to the flexible bag, a drug container held in the capsule and a communicating member for communicating the flexible bag with the drug container. The communicating member has a double-edged hollow needle having a hub in the midway thereof, and a controlling mechanism for controlling the order of communication in such a manner that the plug of the drug container is stuck with one edge of the needle and thereafter the closing film of the flexible bag is stuck with the other end of the needle. The mixing procedure is sure and simple, and can be carried out in a short period of time.