In recent years, model-based development (MBD), a system development method that incorporates simulation technology, has been increasingly adopted in the design of valve-box hydraulic circuits for automatic transmissions. This method has various advantages in terms of development cost and time. On the other hand, hydraulic vibration, response delay, and pressure over/undershoot may occur in actual machine under development that were not easily predicted in the simulation. Although it is necessary to understand the characteristics of the actual machine in order to analyze these problems, the measurement technology has not yet been established. The objective of this research is to develop a measurement technique for hydraulic impedance, which represents the frequency characteristics in the valve box of an automatic transmission, and to identify the parameters. This will not only enable us to evaluate the response and stability of the actual machine, but also to analyze the causes of actual phenomena and improve the accuracy of the model. It will lead to an improvement in design accuracy.