set a collision index value that becomes smaller as a possibility that a host vehicle collides against an object ahead of the host vehicle becomes higher;

determine whether an object recognition condition meets a predetermined condition based on a state of an operation of an operator of the host vehicle by a driver of the host vehicle, the predetermined condition meaning that the driver recognizes presence of the object;

execute warning control for warning the driver when the collision index value is smaller than a first index value;

execute deceleration control for decelerating the host vehicle when the collision index value is smaller than a second index value that is smaller than the first index value and the electronic control unit determines that the object recognition condition does not meet the predetermined condition;

disable execution of the deceleration control when the collision index value is smaller than the second index value and the electronic control unit determines that the object recognition condition meets the predetermined condition;

determine whether the object recognition condition meets the predetermined condition based on a state of an operation of an accelerator operator of the host vehicle and a state of an operation of a brake operator of the host vehicle; and

determine that the object recognition condition does not meet the predetermined condition when the brake operator is not operated and the accelerator operator is operated.