Local Skin friction reduction with microbubbles was measured in a two dimensional channel flow. Microbubbles are expected to reduce the skin friction which takes up a large part of the total ship drag. To adapt the process to a real ship is difficult because of the very large amount of microbubbles. In order to adapt the process to a real ship, we must search for more effective bubble conditions and the persistence of the effectiveness in streamwise direction. The local skin friction reduction was measured in the small high speed water tunnel, systematically changing the averages of void ratio, the main flow velocity and the measurement position from the bubble injection point. The results confirmed the persistence of the effect at the 200δ point (boundary layer thickness) downstream air injection. The local void ratio and the velocity profile using the PIV system were measured. The measurements indicated that the local void ratio near the wall was important to the skin friction reduction.