A highly sensitive photodeflection spectroscopy technique with femtosecond time resolution has been developed. Using this technique, we have measured accurately the shape of acoustic pulses, which were generated from the ultrafast phonon emission in a germanium plate. Supersonic expansion of photoexcited electron-hole plasma was observed. The characteristic velocity of plasma diffusion was evaluated. It exceeded the longitudinal sound velocity in germanium by a factor of 4.0.