[Problem] To provide a mounting structure for an NOx reduction system for a construction machine, said mounting structure being capable of relaxing restrictions on a space for the arrangement of the NOx reduction system.[Solution] In a mounting structure for an NOx reduction system including a urea-water SCR catalyst 19 for reducing and purifying nitrogen oxides emitted from an engine 14, a urea water spay device 20 for spraying urea water into an exhaust passage of the engine 14, a urea water feeder 23 for feeding the urea water to the urea water spray device 20, a first urea-water tank 24 and second urea-water tank 27 for storing the urea water to be guided to the urea water feeder 23, and a controller 30 for controlling drive of the urea water spray device 20 and urea water feeder 23, a counterweight 12 is provided with a cavity 13 formed therein, the urea water feeder 23, first urea-water tank 24, second urea-water tank 27 and controller 30 are integrally arranged as an assembly 21, and the assembly 21 is arranged in the cavity 13 of the counterweight 12.