PROBLEM TO BE SOLVED: To provide a connection device capable of surely maintaining a connection state of a connection member to a body member and securely preventing liquid leakage.SOLUTION: The connection device comprises: a body member 1 which allows circulation of a liquid a connection member A (joint member 2 and screw member 3) which has a connection section 1a formed, the connection section 1a being connectable to the body member 1 and also connectable with a hose for circulating the liquid and a seal member 4 which is arranged between the body member 1 and the connection member A and allows sealing therebetween. The connection device further comprises lock section (catch 6a and locked portion 6b) which holds the body member 1 and the connection member A in a state connected with each other and locks the connection state.