In a hybrid power system, when a change-speed stage selected in one of gear-shift mechanisms for first and second drive trains is switched over to another change-speed stage in the other gear-shift mechanism, deceleration in rotation speed of an engine detected by a rotation speed sensor is compared with a preset upper limit. When the deceleration in rotation speed of the engine exceeds the upper limit, the motor-generator in drive connection with an input shaft of the second drive train is activated as an electric motor to cause load torque canceling or offsetting input torque applied to the input shaft of the other gear-shift mechanism from the engine, and engagement of the change-speed stage in the other gear-shift mechanism is released after the occurrence of the load torque.