A device for supplying an internal combustion engine (1), comprising a carburettor (72) and an air filter (71) installed on an air intake line (70), through which the air filtered by the filter (71) is conveyed into the carburettor (72), and an anti-backflow element (8) located in series on the air intake line (70) between the carburettor (72) and the air filter (71), in which the anti-backflow element (8) exhibits an external casing (80) comprising a perimetral band (81) including at least a portion substantially circular in development closed by two side walls (82, 83), such as to internally define at least a compartment (84) in communication with the carburettor (72) through a duct (85) opening in the perimetral band (81) and developing tangentially relative to the circularly developing portion, the compartment (84) also in communication with the air filter (71) through a first duct (86) opening in one of the side walls (82, 83).