This paper describes the process of developing the parallel mechanism "NINJA ", which is designed to achieve an acceleration rate over 100 [G] (G: gravitational acceleration). To design this mechanism, we introduce a two-step design method (an optimum design method). The technical part of this method consists of formulating the design problem of two steps: Step 1, where we assume and select the design parameters, which should satisfy the required specifications, and Step 2, where we optimize the parameters under the constraint conditions based on the required specifications. To demonstrate the acceleration performance, we present experimental results using the prototype NINJA.