An optical fiber probe system for measuring the local void fraction in the air-water two-phase flow was developed with a 1550 nm light source. Air was injected through a nozzle placed in the center of the bottom wall of a water-filled cylindrical tank. The optical fiber probe having a diameter of $125{mu}m$ was sufficiently thin to resolve the air-water interface of the bubbly flows. To verify the performance of the optical fiber probe, the synchronized high speed visualization study using a high speed camera was carried out. Comparison between the optical signals and the instantaneous bubble diffraction images confirms that the optical fiber probe is very accurate to measure the void fraction in two-phase flows. The estimated bubble diameter and the rising velocity by the optical fiber probe have 1% and 5% of accuracy, respectively.