Catastrophic caldera collapse following a large volcanic eruption or subsurface magmatic outflow is associated with very large displacements on ring faults. The slip resistance or friction on the ring faults is, thus, a key mechanical property that controls the collapse. However, the dynamic friction on the ring faults during caldera collapse remains unknown. Here, we show that dynamic weakening of ring faults caused by frictional melting can play a critical role in the catastrophic collapse of volcanoes. From direct observations of the ring fault of the ancient Jangsan caldera in southeastern Korea, we identified a layer of solidified frictional melt (or pseudotachylyte) measuring ~0.1 m thick. The dynamic friction (expressed as the ratio of shear to normal stress) estimated from the layer, based on high-velocity shear tests and analyses of microstructures and materials, was lower than 0.1. Given the low resistance, it follows that an extraordinarily large fault slip (>100 m) causing a large earthquake was possible during the formation of the ancient caldera. We conclude that the dynamic weakness of ring faults should be incorporated in mechanical models of caldera collapses.