The initial stage of Cs adsorption on a Si(111)7 x 7 surface is studied by means of scanning tunneling microscope (STM) and local tunneling barrier height (LBH) imaging which provides the microscopic work function distribution. Utilizing the combination of STM and LBH imaging, the Cs adsorption features can be classified into three types; clustering, adsorption on an adatom site, and adsorption on a rest atom site. For all these adsorption types, LBH is reduced locally just at the Cs adsorption sites. On the other hand, the increase in the local density of states at the adatom next to the adsorbed Cs is observed. This is considered as a long-ranged effect of Cs adsorption, which has been pointed out in theoretical works.