Abstract: The present invention provides a method of restraining vibration of a floor and an apparatus therefor.According to the method, a reservoir is attached to the floor which is supported from structural members of a floor structure and fluid is put in the reservoir. Geometry of the reservoir and depth of the fluid is determined so that sloshing natural frequency of the fluid coincides with a natural frequency of the floor in a same direction.The vibration restraining apparatus according to the present invention is provided with a reservoir wherein a prescribed amount of liquid is retained. Natural sloshing frequency of the liquid in the reservoir is generally equal to a natural frequency of the floor structure.As the floor vibrates because of vibration of the floor structure or vibrational source on the floor, vibrational energy of the floor is transmitted to the liquid and cause sloshing thereto.