A connection device which connects an end portion of a first information transmission cable (FC1) and an end portion of a second information transmission cable (FC2) to each other, the connection device comprising: a connection section that holds a tubular connector fixing member holding the first information transmission cable (FC1) and having a projection portion on an outer peripheral surface; and a tubular adapter fixing member that holds the second information transmission cable (FC2) and connects the end portion of the first information transmission cable (FC1) and the end portion of the second information transmission cable (FC2) to each other in accordance with insertion of the connector fixing member , wherein the adapter fixing member internally has a guide member being a cylindrical member, wherein the guide member is provided with a guide groove portion formed so as to be parallel to a central axis (CL) of the first information transmission cable (FC1) and the second information transmission cable (FC2) and a helical guide following surface smoothly formed at a predetermined slope angle (BD) in only one direction around the central axis (CL) from a helical vertex portion to a helical rear end portion in order to perform an operation in which the projection portion of the connector fixing member is guided in a manner of gyrating around the central axis (CL), when the connector fixing member is inserted, characterized in that the helical vertex portion and the guide groove portion are adjacent to each other, the helical rear end portion and the guide groove portion are also adjacent to each other, and the guide groove portion is positioned between the helical vertex portion and the helical rear end portion . a connection section that holds a tubular connector fixing member holding the first information transmission cable (FC1) and having a projection portion on an outer peripheral surface; and a tubular adapter fixing member that holds the second information transmission cable (FC2) and connects the end portion of the first information transmission cable (FC1) and the end portion of the second information transmission cable (FC2) to each other in accordance with insertion of the connector fixing member , wherein the adapter fixing member internally has a guide member being a cylindrical member, wherein the guide member is provided with a guide groove portion formed so as to be parallel to a central axis (CL) of the first information transmission cable (FC1) and the second information transmission cable (FC2) and a helical guide following surface smoothly formed at a predetermined slope angle (BD) in only one direction around the central axis (CL) from a helical vertex portion to a helical rear end portion in order to perform an operation in which the projection portion of the connector fixing member is guided in a manner of gyrating around the central axis (CL), when the connector fixing member is inserted, characterized in that the helical vertex portion and the guide groove portion are adjacent to each other, the helical rear end portion and the guide groove portion are also adjacent to each other, and the guide groove portion is positioned between the helical vertex portion and the helical rear end portion . <EOS>