A resonator for internal combustion engines having intake and exhaust pipes connected with a cylinder of said internal combustion engines comprising resonator means for absorbing resonant noises generated in said engines, a plurality of tubular connecting members disposed between said resonator means and one of said pipes for forming a continuous passage from said one pipe to said resonator means, and switch means connected with said tubular connecting means for changing said tubular connecting members in accordance with operational conditions of said engines to absorb said noises from said engines.