PURPOSE: To provide the soundproof plate which decreases noises without increasing the mass of the soundproof plate, is light in weight and has a high effect of decreasing the noises and the soundproof device of a static induction apparatus formed by using this plate.

CONSTITUTION: This soundproof plate 12 for covering a tank in which the body of an electric apparatus is housed is constituted by disposing plural sheets of flat plates 13, 14 apart a narrow spacing and hermetically sealing noncompressible fluid 15 into the spacing therebetween. The apparent pass substantially larger than the actual mass of the soundproof plate 12 is added to the soundproof plate 12 and the vibrations of the flat plate itself, are suppressed owing to the attenuation of the noise by the viscous resistance of the noncompressible fluid. The noises from the tank are thus shut off.