To provide a foundation vibration isolation method of the existing spread foundation structure capable of making vibration isolation also for the existing spread foundation structure not having a margin of soil bearing strength by providing a vibration isolation device to the lower surface of a base footing thereof.

First of all, a first new pile 3 is driven under the existing base footing 2, a part of the ground 1 under the base footing 2 is excavated, a new footing 6 is driven to temporarily provide bearing members 7 after a second new pipe 5 is driven in the excavated section, the rest of the ground under the base footing 2 is excavated and, at the same time, after the first new pile 3 is cut off, new footings 8 are increased to the bottom. After the bearing members 7 are temporarily provided between the said new footings 8 and base footing 2, the bearing members 7 located at a position wherein a vibration isolation device 10 has to be mounted are removed therefor, and other bearing members 7 between the new footing and base footing 2 are removed to bear vertical axial force by the new footing through the vibration isolation device.