A lane keep assist device is configured to perform lane keep assist control for making a host vehicle travel along a lane, and preventing the host vehicle from departing from the lane. The lane keep assist device includes an electronic control device configured to detect presence or absence of another vehicle, present in a vicinity of the host vehicle, which have a gradual decrease in a distance from the host vehicle, and when the other vehicle is detected, the electronic control device configured to set virtual line extending along front-rear direction of the detected other vehicle at position away by first predetermined distance in right-left direction of the other vehicle from lateral side of the detected other vehicle, and to specify the lane based on the set virtual line to perform the lane keep assist control.