To obtain a compact and excellent soundproof effect by a method wherein a piezoelectric actuator is provided on a tank reinforcing member, a vibration with an inversed phase and an almost same level to the vibration with an vibration of the tank reinforcing member is generated by the piezoelectric actuator so as to cancell the vibration of the tank reinforcing member.

A soundproof plate 5 is mounted on a tank reinforcing member 4 through a piezoelectric actuator 6. This soundproof plate 5 is integrally formed by bonding a plurality of steel plates sandwitching sticky and elastic (resin) while using a laminated damping steel plate being made hard to be oscillated. Then, when a frequency of the tank reinforcing member 4 is detected, a control part is made to generate a vibration in an inversed phase to and at an almost same level with the vibration of the tank reinforcing member 4 basing on a detection signal of a vibration detection part to the piezoelectric actuator 6 so as to cancell the vibration of the tank reinforcing member 4 to be propagated to the soundproof plate 5 by means of the piezoelectric actuator 6.