To provide seal rubber capable of improving a sound insulating property at the time of traveling at high speed without increasing the compression force of the seal rubber.

The seal rubber 10 is provided with a flexible fluid enclosing tube 12 arranged in a hollow part 11. The tube 12 is allowed to have a natural resonance frequency different from that of the rubber 10 by enclosing fluidity other than air in the flexible tube similar in material to the rubber 10, reducing noise passing through the rubber 10 among those intruding into the seal part of the door 1 from the outside. When the sound wave of noise reaches the rubber 10, the sound wave out of a specified band, mainly the natural resonance frequency of the rubber 10 is largely absorbed in the wall part of the rubber 10, while the sound wave of the specified band, mainly the natural resonance frequency vibrates the wall of the rubber 10 and passes through it with a small extent of attenuation and reaches the tube 12. Mush of the sound wave is absorbed in the tube 12, since the natural resonance frequency of the tube 12 is made different from that of the rubber 10.