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 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 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 volume comprised in the range of 30% to 99% of the space inside said blood vessel.
