The effect of probe clearance on the rising velocity and chord length of bubbles in a bubble column was examined in measurements,vith the two-channel U-shaped optical fiber probe method. The probe clearances ranged from 0.2 to1.5 cm, A few restrictions required in the signal processing to obtain the reasonable data. With increasing probe clearance, the observed values of the bubble velocities slightly increase; however, those of the bubble chord lengths drastically increase to 160% in the coalesced bubble regime, though they are almost constant in the dispersed bubble regime. A simple theory is proposed considering the probability of bubbles contacting with the probes, and used to successfully explain the experimental results.