Distributions of local droplet parameters such as the droplet fraction, droplet velocity, and droplet diameter were measured using a single optical fiber probe (S-OFP) sensor in a horizontal pipe with an inner diameter of 40 mm and a length of 5 m. Flow condition covers the liquid superficial velocity ranging from 0.01 m/s to 0.015 m/s, and the gas superficial velocity ranging from 27.5 m/s to 32 m/s. Asymmetric distributions of local droplet parameters were observed in the direction of the vertical center line of the test section due to the effect of gravity. The one-dimensional droplet mass flow rate was calculated from distributions of the local droplet fraction and droplet velocity. In this study, the expected maximum height of interfacial wave was considered as a boundary between the droplet and the continuous liquid. In order to validate the droplet mass flow rate measured by the S-OFP sensor, liquid film extraction method was also applied simultaneously in the measuring plane of a test section. It was found that the two methods showed similar results for the droplet mass flow rate under the low liquid flow condition. Whereas, the difference of the droplet mass flow rate between the two methods was large in the high liquid superficial velocity condition. It was resulted by the fact that the liquid film was not completely removed at the liquid film extraction section.