This control method is for a travel control device which is provided with a first detector 110 for detecting obstacles around the local vehicle traveling in a first lane and a second detector 110 fordetecting a second lane adjacent to the first lane, and which, in a lane change target position in the second lane, configures a first range of a size no less than that occupied by the local vehicle on the road surface, detects, as a second area, an area in the second lane which is located to the side of the local vehicle and in which no obstacles are present, and permits the local vehicle to change lanes if the second area includes the first area; this control method involves a step for detecting the travel state of the local vehicle, which includes the position and speed of the local vehicle, a step for acquiring obstacle information including the position and speed of obstacles, a step for predicting the second area after a prescribed time on the basis of the travel state of the local vehicle and the obstacle information, and a step in which, if the second area is predicted to become smaller after the prescribed time, the driver of the local vehicle is notified with notification information that the second area will become smaller.