An assist device (10) for sclerosing treatment of varicose veins, aimed at being partially inserted into a blood vessel (20), including a tubular element (11), which has at least three ways (13, 14, 15), at least two of which (13, 14) are connected each one to at least one related inflatable balloon element (12, 12′, 16) by at least one through hole (30, 31), and at least one remaining way (15) of said three ways (13, 14, 15) has likewise at least one hole (32) for the passage of a sclerosing substance (17), aimed at reaching a wall (18) of said blood vessel (20), in which at least one (16) of said at least two balloon elements (12, 12′, 16) has an elongated shape extending toward at least another balloon element (12, 12′) and a variable volume comprised in the range of 30% to 99% of the space inside said blood vessel, independently from the initial dimensions of the vessel (20), interposed between the external surfaces of the outermost of said at least two balloon elements (12, 12′, 16).