Connection device between a flush-mounted cistern and a water closet, having: a fastening frame having two vertically disposed columns that are disposed on the left and the right; an assembly plate for horizontally connecting the left and the right column, wherein assembly bores for assembling the water closet are provided on the assembly plate; a flushing pipe for connecting the flush-mounted cistern to a flushing line of the water closet; a wastewater pipe for connecting to a wastewater line of the water closet, wherein an entry opening of the wastewater pipe is located below the assembly plate; a guide tube which is designed as a hollow tube, the interior space thereof being used for the installation of cables or water lines; wherein the upper end of the guide tube communicates with the flush-mounted cistern; wherein the lower end of the guide tube is disposed so as to deviate from the central axis by which the fastening frame is divided into a left half and a right half; wherein the guide tube is disposed behind the assembly plate; wherein the fastening frame furthermore has a reinforcement web by way of which the left and the right column are horizontally connected to one another, wherein the reinforcement web is disposed below the entry opening of the wastewater pipe; wherein the guide tube is forked at the bottom so as to form two lower ends; and wherein the guide tube is bent forwards close to its lower end, so that the lower end face is directed towards the front, characterized in that the lower end of the guide tube is disposed below the entry opening of the wastewater pipe; and in that the portion of the guide tube that is bent forwards is locked in place below the reinforcement web. a fastening frame having two vertically disposed columns that are disposed on the left and the right; an assembly plate for horizontally connecting the left and the right column, wherein assembly bores for assembling the water closet are provided on the assembly plate; a flushing pipe for connecting the flush-mounted cistern to a flushing line of the water closet; a wastewater pipe for connecting to a wastewater line of the water closet, wherein an entry opening of the wastewater pipe is located below the assembly plate; a guide tube which is designed as a hollow tube, the interior space thereof being used for the installation of cables or water lines; wherein the upper end of the guide tube communicates with the flush-mounted cistern; wherein the lower end of the guide tube is disposed so as to deviate from the central axis by which the fastening frame is divided into a left half and a right half; wherein the guide tube is disposed behind the assembly plate; wherein the fastening frame furthermore has a reinforcement web by way of which the left and the right column are horizontally connected to one another, wherein the reinforcement web is disposed below the entry opening of the wastewater pipe; wherein the guide tube is forked at the bottom so as to form two lower ends; and wherein the guide tube is bent forwards close to its lower end, so that the lower end face is directed towards the front, characterized in that the lower end of the guide tube is disposed below the entry opening of the wastewater pipe; and in that the portion of the guide tube that is bent forwards is locked in place below the reinforcement web.