The effect of the plasma flow on stability and transport has attracted much interest on fusion plasma research. In the GAMMA 10 tandem mirror, plasmas are mainly produced and heated with Ion Cyclotron Range of Frequency (ICRF) waves. High temperature plasmas are confined in the central cell. Two Mach probes, which have four tips facing direction of every 90 degrees, are installed on east and west sides of the central cell. It is found that the plasma flows from west to east directions with the pitch angle of around 60 degrees. The averaged Mach number of near 0.3 is detected on both sides of the central cell. The driven mechanism of the plasma flow is now under investigation.