Experiments were performed to investigate the heat transfer enhancement against pressure loss by swirling the in-pipe flow in the Reynolds number Re = 1000 ~ 12000. To focus only on the swirl effect, measurements were performed downstream of a swirl generator formed by a twisted tape so that the effect of turbulence due to the tape insert was damped. The spatiotemporal variation of the heat transfer reflecting the vortex motion near the wall was measured by high-speed infrared imaging technique to elucidate the heat transfer enhancement mechanism by swirling the flow. In addition, flow turbulence was measured by laser Doppler velocimetry. By applying the swirl to the flow, the heat transfer coefficient increased against the wall shear stress. In particular, it greatly increased up to 30% at Re ≈ 1500 ~ 2000. The origin of the heat transfer enhancement in the turbulent regime at Re ≈ 5000 is not clear at present, but the experimental data suggest that there are some dissimilarities between momentum and heat transports, which are probably responsible for the heat transfer enhancement. In the laminar regime at Re ≈ 1500 ~ 2000, the transition Reynolds number decreases due to flow instability by swirling the flow. This leads to a large heat transfer enhancement in this regime.