PURPOSE: To restrict the generation of a speed change shock in an automatic transmission, which is connected to a driving source through a retarder, by controlling the braking force of a retarder at the next time of speed change on the basis of a result of the comparison of a target value of the speed change time with a real time thereof, and changing a load to be input to the automatic transmission.

CONSTITUTION: When an automatic transmission 3 is connected to a driving source 1 through a retarder 2, a speed change control means 5 outputs the speed change command on the basis of a parameter, which shows the operation condition of a vehicle from an operation condition detecting means 4, to control a speed change carry-out means 6. At this stage, a speed change time detecting means 8 detects a real value of the speed change time from a start to a finish of speed change, and a comparing means 9 compares this real value with a target value of the speed change time, which is previously stored in a target value storing means 7. In the case where the real speed change time is longer than the target value, a retarder control means 10 raises the braking force of the retarder 2 at the next time of speed change, and on the other hand, in the case where the real speed change time is shorter than the target value, the retarder control means 10 controls so as to reduce the braking force of the retarder 2 at the next time of speed change.