To obtain a sufficient vibration control function without deteriorating the strength of a substrate by forming grooves of a predetermined depth in an annular area a predetermined distance away from a mounting hole in the substrate in a direction toward an outer circumferential portion on both sides of the substrate, respectively.

One 7a of curved portions of a groove 7 in one side 2a of a substrate 2 and the other 8b of curved portions of a groove 8 in the other side of the substrate are located substantially at the same position, and the other curved portion 7b of the groove 7 and one curved portion 8a of the groove are also located substantially at the same position. In the grooves 7, 8 on the front side 2a of the substrate and the other side thereof, arc portions 7c, 8c are provided in such a manner as to be continous on a circumference whose diameter is r1 and to be located at displaced positions, and an intersecting portion of tip end portion, central portion and lower end portion of the respective curved portions 7a, 7b, 8a, 8b is put through to a side of the substrate opposite to the front side 2a thereof. Impact vibrations alternately applied to the front side 2a and opposite side of the substrate 2 when it is rotated are caused to act in directions in which they are cancelled by each other owing to the grooves 7, 8, thereby making it possible to obtain a sufficient vibration control function by effectively damping impact vibrations without deteriorating the strength of the substrate.