A composite planetary transmission section (P) is provided for synthesizing output from a stepless speed changing section (20) with an engine drive force. In a transmission line from the planetary transmission section (P) to an output rotational body (90), there are provided first clutch mechanism (60), a second clutch mechanism (70), a speed-reducing planetary transmission mechanism (80), an operable coupling clutch mechanism (110) and an output clutch mechanism (120). A ring gear (83) of the speed-reducing planetary transmission mechanism (80) includes a brake mechanism (100). A sun gear (43) of the planetary transmission section (P), an input side rotational member (62) of the first clutch mechanism (60), an input side rotational member (71) of the second clutch mechanism (70), a sun gear (84) of the speed-reducing planetary transmission mechanism (80), and an input side rotational member (122) of the output clutch mechanism (120) are rotatable about a common rotational axis. A rotational shaft (97) operably coupling a carrier (44) of the composite planetary transmission section (P) with the output rotational body (90) is inserted through a planetary transmission mechanism (50) of the composite planetary transmission section (P), the first clutch mechanism (60), the second clutch mechanism (70) and the speed-reducing planetary transmission mechanism (80).