A driving support Electronic Control Unit (ECU) initializes a target trajectory calculation parameter at a start of Lane Change Assist Control (LCA), calculates, based on the target trajectory calculation parameter, a target trajectory function representing a target lateral position in accordance with an elapsed time from the start of LCA; and calculates a target control amount according to the target trajectory function. When it is determined that the own vehicle has crossed a boundary white line, the driving support ECU again initializes the target trajectory calculation parameter, and calculate the target trajectory function based on the target trajectory calculation parameter.