1 fig7 a to 7c show respective measurement results for the boundary microphone 1 using the boundary plate 2 having the cross section shown in fig6 note that dimensions of the boundary plate 2 and arrangement of the condenser microphone 3 here are as exemplified in fig1 and 2 also respective characteristics indicated by symbols a to d in fig7 a are obtained under the same condition as the measurements indicated by symbols a to d in fig4 a similarly respective characteristics indicated by symbols a to c in fig7 b are obtained under the same condition as the measurements indicated by symbols a to c in fig4 b when comparing a frequency characteristic where a sound source is at 90 u00b0 as indicated by symbol b in fig7 a and a frequency characteristic where the sound source is also at 90 u00b0 as indicated by symbol b in fig4 a the characteristic indicated by symbol b in fig7 a shows that degradation of directional frequency response due to vibration of the boundary plate 2 itself is greatly reduced in other words the characteristic indicated by symbol b in fig7 a is closer to the characteristic of the microphone alone as indicated by symbol b in fig3 a this results from the fact that the boundary plate 2 of the embodiment has a favorable vibration damping property thus phase interference of sound signals due to vibration of the boundary plate 2 itself is greatly reduced furthermore frequency characteristics where the sound source is.