To provide a connection structure or the like capable of facilitating strong connection and disconnection between a catheter for enteral nutrition and a medical circuit exclusive for the catheter, and also capable of connecting the catheter for enteral nutrition and various kinds of male connectors.

A connection structure between a catheter for enteral nutrition and a medical circuit, includes a male connector 2 connected to one end of a transfer tube 41 and provided with female threads 213, and an adapter 3 freely detachably mounted on one end of a tube 51 and provided with male threads 308 threadedly engaged with the female threads 213, wherein by the thread engagement between the female threads 213 and the male threads 308, the outer side member 303 of the adapter 3 is elastically deformed so as to approach an axis of the tube 51, and the tube 51 is held by the inner side member 302 and outer side member 303 of the adapter 3.