An experimental investigation was carried out for the buoyancy-driven exchange flow in a narrow vertical pipe concerning the air ingress process during a standpipe rupture in a high-temperature gas-cooled reactor. In the current study, the evaluation method of exchange flow was developed by measuring the velocity in the pipe using a laser Doppler velocimeter. The experiments were performed under atmospheric pressure with nitrogen as a working fluid. The Rayleigh numbers range from 2.0 × 104 to 2.1 × 105. The exchange flow fluctuated irregularly with time and space in the pipe. It was found that the exchange-velocity distribution along the horizontal axis changed from one- to two-humped curves with increasing Rayleigh number. In the case that the lower plenum wall was cooler than the heated disk, the volumetric exchange flow rate was smaller than that in the case where the lower plenum wall and heated disk were kept at the same temperature.