A muffler which reduces engine noise at lower operating speeds includes two spacers installed within a cylinder that is divided into a front chamber, a resonance canceling chamber and a noise canceling chamber. A first pressure returning tube and a second pressure returning tube are serially connected within the cylinder and a plurality of gas holes are circularly installed within the first pressure returning tube and the second pressure returning tube so that the first pressure returning tube and the second pressure returning tube pass through the front chamber, resonance cancelling chamber and noise cancelling chamber. A spacer circularly installed with a plurality of through holes is installed within the front chamber and the latter is divided into an expansion chamber and an air pressure regulating chamber. A central hole is located in the center of the spacer. The front end of the first pressure returning tube is placed in the central hole of the spacer and a gap is formed between the front end of first pressure returning tube and the central hole of the spacer.