Abstract: Device cooperating with a vehicle safety belt, designed for passenger transport, especially children consists of a tensioning belt (1) fixed releasably between the shoulder part (2) and the lap part (3) of the fastened safety belt. Tensioning belt is equipped with the length and tension controller (R) and has tunnels (9) on both ends formed by the rolled up and fixed ends of the belt. In the tunnels (9) there are arms of the snap fasteners (4) fixed to the fastened safety belt. Controller (R) includes a staple (7) fixing the length of the tensioning, belt (1) and a staple (8) blocking its position, and the tensioning belt (1) is composed of a fragment (1?) connected permanently with the fixing staple and a fragment (1?) connected slidingly with the fixing staple (7) and the staple (8) blocking the position of the belt (1).