This paper investigates a bubble, placed so that it bridges the cross section of a horizontal narrow divergent passage under Earth gravity (1-G). In a narrow passage, inertial forces are known to be small in comparison with viscous forces. Gravitational force is not dominant in bubble behavior in a horizontal narrow passage under the 1-G condition. In this sense, the bubble behavior in the passage is similar to that under a low-gravity condition. Understanding bubble behavior is important in relation to separating a gas from a two-phase gas-liquid flow and controlling a gas-liquid interface under low-gravity conditions. Thus, a single bubble is placed under the 1-G condition. The bubble geometry and its behavior are studied experimentally for gap sizes ranging from 0.5 mm to 2 mm and divergent angles from 1° to 5° using ethyl alcohol as the working fluid. The following results were obtained: (1) the bubble was found to move to the greater cross-sectional area; (2) the gas-liquid interface geometry in a top view can be expressed as a contact circle model that takes the maximum radius in the passage; and (3) the effects of gap size and the projected bubble area on bubble behavior are clarified.