PROBLEM TO BE SOLVED: To provide a releasable connection structure which facilitates connection between members and its release and facilitates cleaning, and a medical instrument having the connection structure.SOLUTION: A connection structure 30 releasably connects a guide tube of forceps and a distal-end medical implement and the like. The connection structure 30 comprises a connection shaft 32, a chuck 34, and a fixing sleeve 36. The fixing sleeve is fitted around a flexible part 32b of the chuck in a state where a locking part 34c of the chuck is locked between first and second projections 32d, 32e of the connection shaft, the flexible part is held by a blocking surface 36d, and the chuck 34 and the shaft 32 are connected while maintaining a state where a first locking surface 32f of the connection shaft and a second locking surface 34g of the chuck are engaged. When the fixing sleeve 36 is pulled distally from the connected state, a fourth locking surface 36f of the fixing sleeve pushes a third locking surface 34h of the chuck to bend the flexible part inward, and thus an engagement between a locking groove 36e and a locking projection 34e is released.SELECTED DRAWING: Figure 4