A speed change transmission apparatus comprising: a hydrostatic stepless speed change section having a hydraulic pump and a hydraulic motor and receiving an output of an engine via a pump shaft; a planetary transmission section having a pair of output members, the planetary transmission section being configured to combine a drive force outputted from the hydrostatic stepless speed change section and an engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section and to output the combined force from said pair of output members; and a speed change output section configured to output combined drives force outputted from the pair of output members in a plurality of speed ranges; said planetary transmission section including: a first planetary transmission mechanism having a first sun gear, a first ring gear and a first carrier and configured to receive the output from said hydrostatic stepless speed change section and the engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section; and a second planetary transmission mechanism having a second sun gear, a second ring gear and a second carrier and configured to receive output from said first planetary transmission mechanism (P 1); wherein said second sun gear of said second planetary transmission mechanism is operatively coupled to one of said pair of output members; and a power takeoff shaft is operatively coupled with the pump shaft; characterized in that the output from the hydrostatic stepless speed change section is inputted to the first sun gear of the first planetary transmission mechanism and the engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section is inputted to the first carrier; the second ring gear of the second planetary transmission mechanism is operatively coupled to the first sun gear of the first planetary transmission mechanism (P 1) to be rotatable in unison therewith, the second sun gear is operatively coupled to the first ring gear of the first planetary transmission mechanism (P 1) to be rotatable in unison therewith; and the second carrier of the second planetary transmission mechanism is operatively coupled to the other of the pair of output members. a hydrostatic stepless speed change section having a hydraulic pump and a hydraulic motor and receiving an output of an engine via a pump shaft; a planetary transmission section having a pair of output members, the planetary transmission section being configured to combine a drive force outputted from the hydrostatic stepless speed change section and an engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section and to output the combined force from said pair of output members; and a speed change output section configured to output combined drives force outputted from the pair of output members in a plurality of speed ranges; said planetary transmission section including: a first planetary transmission mechanism having a first sun gear, a first ring gear and a first carrier and configured to receive the output from said hydrostatic stepless speed change section and the engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section; and a second planetary transmission mechanism having a second sun gear, a second ring gear and a second carrier and configured to receive output from said first planetary transmission mechanism (P 1); a first planetary transmission mechanism having a first sun gear, a first ring gear and a first carrier and configured to receive the output from said hydrostatic stepless speed change section and the engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section; and a second planetary transmission mechanism having a second sun gear, a second ring gear and a second carrier and configured to receive output from said first planetary transmission mechanism (P 1); wherein said second sun gear of said second planetary transmission mechanism is operatively coupled to one of said pair of output members; and a power takeoff shaft is operatively coupled with the pump shaft; characterized in that the output from the hydrostatic stepless speed change section is inputted to the first sun gear of the first planetary transmission mechanism and the engine drive force that has not been subjected to any speed change action by the hydrostatic stepless speed change section is inputted to the first carrier; the second ring gear of the second planetary transmission mechanism is operatively coupled to the first sun gear of the first planetary transmission mechanism (P 1) to be rotatable in unison therewith, the second sun gear is operatively coupled to the first ring gear of the first planetary transmission mechanism (P 1) to be rotatable in unison therewith; and the second carrier of the second planetary transmission mechanism is operatively coupled to the other of the pair of output members.