Recent studies have found various factors that facilitate “organizational citizenship behavior”, which is one of the voluntary behaviors among employees. Here, we focus on servant leadership and trust in managers. We investigated (a) whether servant leadership affects employee’s trust in managers and (b) whether employee’s trust in his/her manager mediates the effect of servant leadership by managers upon organizational citizenship behaviors. A survey was administered to 108 employees who work for 12 different shops of a restaurant chain. Results of structural equation modeling and mediation analyses by the bootstrap method showed that (a) servant leadership was a determining factor in employee’s trust in managers and that (b) employee’s trust in his/her manager mediated the process of servant leadership, facilitating the “civil virtue” component of organizational citizenship behavior.