A travel assistance device configured to execute a travel assistance for platooning of vehicles that form a vehicle group, the travel assistance device comprising: a first control unit (S103, S201, S601) configured to execute lane changing of a rearmost vehicle from a first lane to a destination lane when a lane changing instruction for the vehicle group has been issued, the rearmost vehicle being a vehicle located at a rearmost part of the vehicles that form the vehicle group; and a second control unit (S203 to S207, S603 to S607) configured to permit lane changing of at least one vehicle (41 to 43) that forms the vehicle group other than the rearmost vehicle from the first lane to the destination lane after executing the lane changing of the rearmost vehicle; wherein standby traveling includes traveling in which the rearmost vehicle follows in the destination lane and in the traveling direction of the vehicle group the vehicles (41 to 43) that form the vehicle group other than the rearmost vehicle, and the travel assistance device comprises a standby traveling control unit (S204, S212, S213, S406, S412, S712, S720) configured to cause the rearmost vehicle to execute the standby traveling during a period from the execution of the lane changing of the rearmost vehicle to completion of lane changing of an entirety of the vehicle group; characterized in that a distance between the rearmost vehicle and a lead vehicle in the traveling direction of the vehicle group during the standby traveling is a first distance, the lead vehicle being a vehicle of the vehicles forming the vehicle group that is closest to the rearmost vehicle, a distance between the rearmost vehicle and the lead vehicle in the traveling direction prior to the lane changing instruction of the vehicle group is a second distance, and the standby traveling control unit is configured to cause the rearmost vehicle to execute the standby traveling such that the first distance becomes shorter than the second distance. a first control unit (S103, S201, S601) configured to execute lane changing of a rearmost vehicle from a first lane to a destination lane when a lane changing instruction for the vehicle group has been issued, the rearmost vehicle being a vehicle located at a rearmost part of the vehicles that form the vehicle group; and a second control unit (S203 to S207, S603 to S607) configured to permit lane changing of at least one vehicle (41 to 43) that forms the vehicle group other than the rearmost vehicle from the first lane to the destination lane after executing the lane changing of the rearmost vehicle; wherein standby traveling includes traveling in which the rearmost vehicle follows in the destination lane and in the traveling direction of the vehicle group the vehicles (41 to 43) that form the vehicle group other than the rearmost vehicle, and the travel assistance device comprises a standby traveling control unit (S204, S212, S213, S406, S412, S712, S720) configured to cause the rearmost vehicle to execute the standby traveling during a period from the execution of the lane changing of the rearmost vehicle to completion of lane changing of an entirety of the vehicle group; characterized in that a distance between the rearmost vehicle and a lead vehicle in the traveling direction of the vehicle group during the standby traveling is a first distance, the lead vehicle being a vehicle of the vehicles forming the vehicle group that is closest to the rearmost vehicle, a distance between the rearmost vehicle and the lead vehicle in the traveling direction prior to the lane changing instruction of the vehicle group is a second distance, and the standby traveling control unit is configured to cause the rearmost vehicle to execute the standby traveling such that the first distance becomes shorter than the second distance.