The expansion tube is known as a simple facility realizing a high enthalpy flow and has been employed in a variety of applications ranging from a super-orbital reentry capsule to an electrodynamic heat shield technique. In this study, the flow generated in the expansion tube was experimentally and theoretically investigated to evaluate the test time period and the test flow condition realized in the expansion tube about several flow conditions. The pitot pressure was measured at the test section and the secondary pitot pressure jump which indicates the contact surface arrival was observed. This fact is also confirmed through observations of the flow field around the pitot pressure probe. For the highest flow velocity case, the simple theoretical method assuming equilibrium flow gives a good estimation about flow conditions and test time. However, for the other cases, this estimation method must be improved.