The optical properties of line-defect waveguides in two-dimensional photonic crystal slabs are investigated using picosecond light pulses. Time-domain waveforms of the light pulse propagating through the waveguide are successfully observed using an autocorrelation method. The group velocity of the waveguide is directly determined from the group delay time for light pulses reflected back and forth along the waveguide. A small group velocity of one-twentieth the speed of light in vacuum is observed at a frequency near the edge of the waveguide mode. The frequency dependence of the group velocity is also measured, and the group-velocity dispersion is found to be larger than that of normal single-mode optical fibers by a factor of 10 4-10 5 .