Effects of a flow obstacle are examined on the film thickness in gas-liquid two-phase upward flow belonging to slug, froth and annular flow regimes. In this experiment, a ring type obstacle is employed as the obstacle, and time varying holdup is measured by means of the constant electric current method at eight cross sections near the obstacle. Discussions are made mainly on the minimum film thickness determined by the measured holdup. Whether the obstacle is inserted or not makes essential difference in the process of changing film thickness. Values of minimum film thickness for the upstream zone of the obstacle take minima at rather small air velocities and are reduced to approximately a tenth as large as those without obstacle. Minimum film thickness in the downstream zone decreases monotonously with air velocity.