PURPOSE: To suppress vibration of a main engine (sound source equipment) of a ship from propagating to a radiating surface (ship bottom shell plate) to underwater, in the ship such as an oceanic research ship of mounting an observation acoustic equipment.

CONSTITUTION: Tank structure walls 9, 9a are mounted under a double bottom top plate 5, on which a machine base 4 of a main engine 3 is mounted, to form tank sections 8, 8a, and further plate thickness of the tank structure walls 9, 9a is set so as to maximize an acoustic radiation loss in a sonar frequency range. In this way, vibration energy, generated in the main engine 3, is radiated as acoustic energy into the tank sections 8, 8a, to suppress propagation to a ship bottom shell plate 6 and also so as to obtain a large vibration energy loss in the vicinity of the sonar frequency range by the tank structure walls 9, 9a.