We propose the assist system for the sidewalk snow remover operation that makes sidewalk snow removal easier and safer. Currently sidewalk snow removal is carried out by compact rotary snow blowers. The assist system provides information on guide routes and warnings to the vehicle drivers. The system always recognizes the vehicle's position and provides assisting information corresponding to the position. Therefore, in order to assist effectively, it is essential for the assist system to detect the vehicle's position with high precision. So, we have been developing the detection method of vehicles with the radio wave marker sensor. The sensor can measure the positions of vehicles even in an environment with snow and water. This paper explains the outline of the assist system and the details of the detection method of the vehicles, and shows the validity of this method with results from experiments and simulations.