A connection device includes a main holder including a syringe holder to be attached to a syringe and a cannula holder which includes a double-head cannula. The cannula holder is located at a retracted position retracted with respect to the syringe holder in the pre-use state, and is advanced to an advanced position abutting on the syringe holder in the use state. The connection device further includes a separation preventing mechanism which engages the syringe to prevent separation of the syringe from the syringe holder in the state where the cannula holder is located at the retracted position. When the cannula holder is located at the advanced position, the separation preventing mechanism releases an engagement state with the syringe to permit the separation of the syringe from the syringe holder.