To protect the health of subway users, it is essential to investigate the generation, resuspension, and decay of particles in underground tunnels. In this paper, we analyzed the diurnal pattern of variation in particle concentration in a subway tunnel. The mass concentration of particles was measured in a shelter located midway in the underground tunnel connecting Janghanpyeong station and Gunja station on Seoul Subway Line 5 by using an aerodynamic particle sizer, a dust monitor, and a fast mobility particle sizer. The particle mass concentration increased uniformly as a train passed through the tunnel, which was followed by an exponential decay. The particle concentration in the tunnel decreased when the decrease in the number of particles between train operations exceeded the number of particles generated and resuspended by a passing train, and increased in the opposite case. The diurnal variation in particle concentration in the tunnel according to train operations can be predicted by applying such a pattern of variation to the mass concentration of particles.