To systematize parallelism of plates and mounting of them by arranging building and footing ground on both up and down plates to generate pressure in a viscous fluid caused by load, and sliding two plates each other under the balance between capacity of an accumulator and pressure of sealed gas.

A sliding base isolation element is constituted of O-ring shaped seal members 3 and 4 for sealing the periphery between upper plate 1 and lower plate 2, a viscous fluid 5 sealed between them and accumulator 6. One or the other of both up and down plates includes a fluid reservoir, capacity of the fluid is increased as a whole, and for example, accumulator, etc., are mounted between footing and building. Gas having appropriate pressure is sealed into each of the accumulators, and load imposed on the accumulator 6 keeps its balance. Accordingly, friction between seal members 3, 4 and up and down plates 1, 2 is adjusted by adjusting pressure of the accumulator 6, etc., and influence by wind, etc., is controlled to set. By the constitution, it is effective to lightweight load structure such as low-rise residence, etc., mechanism is simplified to have reliability on trouble and elapsed variation therein, and maintenance can be easily executed.