A hybrid drive system comprising a first drive machine as an internal combustion engine, and a second drive machine as an electric engine, for a motor vehicle, comprising: a first gearshift sub-transmission having a first input shaft and being directly coupled to an output shaft by means of a first group of shifting gearwheel pairs (41/51, 42/52, 43/53, 44/54), wherein the shifting gearwheel pairs (41/51, 42/52, 43/53, 44/54) of the first group each have a gearwheel (41, 42, 43, 44) on the first input shaft and a gearwheel (51, 52, 53, 54) on the output shaft, and wherein the first input shaft is coupled to the first drive machine by means of a friction clutch, a second gearshift sub-transmission having a second input shaft and being directly coupled to the output shaft by means of a second group of shifting gearwheel pairs (45/51, 46/52, 47/53, 48/54), wherein the shifting gearwheel pairs (45/51, 46/52, 47/53, 48/54) of the second group each have a gearwheel (45, 46, 47) on the second input shaft and a gearwheel (51, 52, 53) on the output shaft, and wherein the second input shaft is in a drive connection with the second drive machine, characterized in that at least two of the shifting gearwheel pairs (41/51, 42/52) of said first group of shifting gearwheel pairs (41/51, 42/52, 43/53, 44/54) are formed exclusively by engaging and disengaging idler gears (41, 42) on the first input shaft and by engaging and disengaging idler gears (51, 52) on the output shaft, wherein the engaging and disengaging idler gears (41, 42) on the first input shaft can be coupled to the first input shaft by switching elements (A, C) and the engaging and disengaging idler gears (51, 52) on the output shaft can be coupled to the output shaft by the switching elements (A, C). a first gearshift sub-transmission having a first input shaft and being directly coupled to an output shaft by means of a first group of shifting gearwheel pairs (41/51, 42/52, 43/53, 44/54), wherein the shifting gearwheel pairs (41/51, 42/52, 43/53, 44/54) of the first group each have a gearwheel (41, 42, 43, 44) on the first input shaft and a gearwheel (51, 52, 53, 54) on the output shaft, and wherein the first input shaft is coupled to the first drive machine by means of a friction clutch, a second gearshift sub-transmission having a second input shaft and being directly coupled to the output shaft by means of a second group of shifting gearwheel pairs (45/51, 46/52, 47/53, 48/54), wherein the shifting gearwheel pairs (45/51, 46/52, 47/53, 48/54) of the second group each have a gearwheel (45, 46, 47) on the second input shaft and a gearwheel (51, 52, 53) on the output shaft, and wherein the second input shaft is in a drive connection with the second drive machine, characterized in that at least two of the shifting gearwheel pairs (41/51, 42/52) of said first group of shifting gearwheel pairs (41/51, 42/52, 43/53, 44/54) are formed exclusively by engaging and disengaging idler gears (41, 42) on the first input shaft and by engaging and disengaging idler gears (51, 52) on the output shaft, wherein the engaging and disengaging idler gears (41, 42) on the first input shaft can be coupled to the first input shaft by switching elements (A, C) and the engaging and disengaging idler gears (51, 52) on the output shaft can be coupled to the output shaft by the switching elements (A, C).