We carried out the net impulse measurements generated from the detonation tube filled with stoichiometric hydrogen-air mixture at an atmospheric pressure and a room temperature. In order to accurate measurement involves the outflow of unburned mixture, a diaphragm-less detonation tube was used. The net impulse measured directly by using the ballistic pendulum apparatus and pressure-based value obtained from thrust wall pressure at close end of the tube. Obstacles which promote deflagration-to-detonation transition, DDT, were arranged in varied blockage ratio, BR and varied length respectively. From the result of a series of experiments that BR taken as a parameter, DDT occurred in the range of BR = 0.17-0.43. The specific impulse has maximum value under the condition of BR ≅ 0.2. It was obtained minimized of the momentum losses from the comparison with theoretical value calculated based on simplified PDE model [Endo, T., Kasahara, J., and Fujiwara, T., "Pressure History at the Thrust Wall of a Simplified Pulse Detonation Engine", AIAA Journal, Vol.42, No.9, pp. 1921-1930, 2004.]. From the comparison of net impulse between DDT and no DDT condition, it was confirmed to obtain high thrust in constant volume combustion, CVC, mode. Impulse losses under the condition of lower BR mainly due to outflow of unburned mass induced by pressure waves ahead of a flame front.