Current-voltage characteristics of drifting spherical Langmuir probes have been studied experimentally in collisionless plasmas. In order to single out the effect of probe speed on the ion current collection, the experiment was carried out by a rotating arm facility. With this device the probe speed as well as the properties of the undisturbed plasma were accurately determined. As predicted by the theories of Kanal, Godard, and Laframboise, the ion current at small speed ratios was found to be either increased or decreased as the probe speed increased depending on the ratio of probe radius to Debye length R/λD. As for the absolute magnitude of ion current change by the probe speed, however, discrepancies were found between these theories and the present experiment. The difference is considered to originate from asymmetry in the potential distribution, which is not included in these theories.