A vibration detector of the seismic system comprisesa weight member;a support frame;at least two weight supporting members, each of the supporting members being fixed to the weight member at one end thereof and being substantially fixed to the support frame at one portion thereof in detecting the vibration; anda member for substantially fixing one portion to the support frame in detecting the vibration; wherein the weight member is vibratively supported by the tension of the supporting members, and at least a part of one of the supporting members is formed by a piezoelectric polymeric substance having a pair of vibration detecting electrodes, the part being at a position between the fixed end and one substantially fixed portion of the supporting member.