Mechanism of the occurrence of wave breaking with vortex sheets is discussed by thinking the self-similar flow in strict consideration of the gravitational effect. Approximate realization of the self-similar flow with wave breaking was experimentally confirmed by using a ship model with triangular similar sections whose keel line varies as AX2, where X is the longitudinal length. It is shown that the zonal breadth of wave breaking is proportional to ε X2/F2 and it is related to the zone of the existence of longitudinal waves as a result of applying a linearized theory under the assumption that ε<<1 and F<<1, where ε is an appex angle and F is defined as U∞√A/g. Examinations of local solutions under the nonlinear free surface conditions showed an existence of the singular solution with a vortex sheet on the production of folded wave surface into two. Its solution explains the mechanism of the initiation of wave breaking phenomena.