[en] In an FBR type reactor, a main container supporting skirt is disposed on a base mat of a reactor building and, in addition, a vibration proof device having thin layerous rubbers and steel plates disposed alternately and also having flanges at the upper and the lower portions is disposed between the base mat of the reactor building and a lower raft secured to the rock substrate. For the inputted seismic vibrations in the horizontal direction, since the period of horizontal movements of the building is made greater than the period of the seismic movement by the vibration proof device, impact shocks of earthquakes can be moderated. Further, the inputted vertical seismic movements are not amplified but sufficiently moderated during transmission from the base mat to the reactor structure by way of the main container support skirt, because the load transmission path is short. Since the loads of equipments in a main container are not applied to a reactor container cover, the thickness of the structures can be decreased and the temperature gradient can be reduced, which is effective for reducing thermal stresses. (N.H.)