The temperature dependence of the c 33 longitudinal acoustic modes of ferroelectric TGS and TGSe is observed in the time domain by impulsive stimulated Brillouin scattering (ISBS). The temperature dependence of the acoustic velocity is measured near T c (49°C for TGS and 22°C for TGSe). The elastic anomaly due to the coupling between the acoustic mode and the electric polarization fluctuation is observed with frequency near 2.0 GHz, where the conventional methods, ultrasonic and Brillouin scattering, are usually difficult to utilize. The anomaly is interpreted phenomenologically to be caused by the critical slowing down of the electric polarization fluctuation. The relaxation time of the electric polarization fluctuation is discussed.