To provide a characteristic frequency measuring system capable of measuring the characteristic frequency of a building instantly and simply when the measurement is required.

This characteristic frequency measuring system 1 is constituted of an excitation means 2A(2) constituted of a plate 21 capable of reciprocating slide and stoppers 22, 22 provided on the end of a domain L capable of reciprocating slide, and a vibration detection means 4A(4) equipped with a plurality of such tense strings 41 between two supporting points that each string 41, etc., has a different characteristic frequency. A person N gets on the plate 21 and performs a reciprocating slide movement repeatedly in the X1 direction and in the Y1 direction alternately, to thereby generate horizontal forces F1, F2, and the building 8 is vibrated in the X2 direction and in the Y2 direction alternately, and a response vibration of the building 8 is measured by the vibration detection means 4A(4), to thereby determine the characteristic frequency of the building 8.