A vehicle body reinforcement structure includes an elongated hollow structure formed by jointing an outer panel and an inner panel with each other, and a reinforcement member configured to partition the hollow structure into a lower closed cross section and an upper closed cross section. At inside of at least one of the lower closed cross section and the upper closed cross section, at least a pair of bulkheads extending so as to cross the closed cross section are disposed so as to be separated with each other in a direction in which the closed cross section is extending. A space between the pair of bulkheads is made as a filler chamber which is filled with filler. A filling opening for the filler is provided on the inner panel so that the filling opening is in communication with the filler chamber.