First developed for manufacturing context of the Toyota Production System, the Lean approach has been extended to product development about 20 years ago. Lean Product Development approach emphasizes on a method to implement a flowing development process. However, some tasks require decision milestones that can interrupt this continuous process flow. The paper focuses on safety engineering, an aspect which requires decision milestones due to standardized validation processes. It proposes a specific way to integrate safety constraints into a continuous and flowing process of Lean Product Development. To perform the safety integration in Lean Product Development, the proposed method simultaneously addresses process improvement of both product development and safety control. The proposal is a five-step method addressing the whole product development process from the requirements engineering step to the validation of the product prototype. This method was tested on an industrial case study and implemented into PLM TeamCenter.