  High resolution imaging observation of a sunspot umbra was done with Hinode
Solar Optical Telescope (SOT). Filtergrams in wavelengths of blue and green
continuum were taken during three consecutive days. The umbra consisted of a
dark core region, several diffuse components and numerous umbral dots. We
derived basic properties of umbral dots (UDs), especially their temperatures,
lifetimes, proper motions, spatial distribution and morphological evolution.
Brightness of UDs is confirmed to depend on the brightness of their surrounding
background. Several UDs show fission and fusion. Thanks to the stable condition
of space observation, we could first follow the temporal behavior of these
events. The derived properties of internal structure of the umbra are discussed
in viewpoint of magnetoconvection in a strong magnetic field.
