Next generation on-chip optical devices require light manipulation in time and space, that is, control of group velocity of light in subwavelength dimensions. A waveguide in plasmonic crystal fulfills such requirements offering nanoscale light confinement in the dispersion-tunable plasmonic crystal matrix. However, there has been no direct access to the local dispersion of the waveguide mode itself, and the group velocity of light could not be evaluated. Herein, for the first time, we experimentally clarify the dispersion of the waveguide modes by use of angle-resolved cathodoluminescence scanning transmission electron microscopy. Their group velocity can be extremely slowed down by the existence of a bandgap formed in the waveguide in the energy range of the plasmonic crystal bandgap.