A collision avoidance ECU sets a model deceleration change amount to smaller value in a state in which it is difficult to reduce the speed of a host vehicle than in a state in which it is easy to reduce the speed of the host vehicle. The collision avoidance ECU calculates a first target value by multiplying the model deceleration change amount by the elapsed time. The collision avoidance ECU obtains a subtraction value by subtracting the current reference relative deceleration from the first target value. Then, the collision avoidance ECU determines a target relative deceleration to be a greater value when the subtraction value is large than when the subtraction value is small, and carries out brake control so that the reference relative deceleration approaches the target relative deceleration.