Abstract In this paper, the wake fields reaction force, which acts on electrons in an accelerator cavity, is discussed. The first part of this paper concerns an analysis method of the wake fields. The calculation method of the wake fields is based on the Boundary Element Method (BEM). The application of BEM to this analysis enables us to consider the separation of the electromagnetic fields into two parts, the self-fields of the electrons and the induced fields (wake fields). In the second part of this paper, the reaction force for the electrons, which is caused by the wake fields, is considered using the results of the wake fields analysis. It is found from the analysis that the reaction force strongly depends on the position in the electron bunch. This predicts that wake fields transform the bunch shape drastically. It should be also noted that BEM has a great advantage for wake fields analysis compared with FDM or FEM, because it is not necessary to treat singularities of electromagnetic fields around the charged particles.