Abstract: Higher-order vibration is controlled in an event that an impact load such as an aircraft impact is applied to a nuclear plant. A higher-order vibration control device 1 is installed in a nuclear plant having a reactor containment vessel and a nuclear reactor building 3. The higher-order vibration control device 1 includes an impactor 1a, a housing 1b which receives the reaction force of the impactor 1a, and a locking mechanism 2. The impactor 1a is installed on a floor 31 of the nuclear plant so as to roll in a horizontal direction with respect to the floor 31. The housing 1b encloses the impactor 1a and guides rolling of the impactor 1a. The locking mechanism 2 restrains rolling of the impactor 1a. In the event that a flying object may possibly impact the nuclear plant, the locking of the locking mechanism 2 is released.