The objective of this study is to elucidate the mechanisms of bubble coalescence. The coalescence of horizontally contacting twin bubbles is experimentally investigated using an isothermal air-water system. The liquid film thickness formed between the contacting bubbles is measured using an improved laser extinction method. The variation of liquid film thickness between the bubbles at the rupture location and the distribution of the liquid film thickness are evaluated. The experimental parameters are the airflow rate and the measurement position (transmitted laser position). The bubble approach velocity and the time from the start of collision until coalescence were measured. When bubble coalescence occurs rapidly, the liquid film thickness is thinnest near the center, and this position moves toward the periphery from the center with increasing contact duration. The thinnest film thickness appeared just before coalescence and was approximately 1.0 mu m. A ring-shaped thinner area in the liquid film emerged and shifted from the center toward the periphery of the liquid film with the increase in the bubble approach velocity and close contact duration. The thinnest liquid film thickness just before the commencement of rupture in the ring-shaped area was approximately 0.3 mu m.