When a planet forms a deep gap in a protoplanetary disk, dust grains cannot pass through the gap. As a consequence, the density of the dust grains can increase up to the same level of the density of the gas at the outer edge. The feedback on the gas from the drifting dust grains is not negligible in such a dusty region. We carried out two-dimensional two-fluid (gas and dust) hydrodynamic simulations. We found that when the radial flow of the dust grains across the gap is halted, a broad ring of dust grains can be formed because of the dust feedback and the diffusion of the dust grains. The minimum mass of the planet needed to form the broad dust ring is consistent with the pebble-isolation mass in the parameter range of our simulations. The broad ring of dust grains is a good environment for the formation of the protoplanetary solid core. If the ring is formed in the disk around a Sun-like star at∼ 2 au, a massive, solid core (∼ 50 M⊕) can be formed within the ring, which may be connected to the formation of hot Jupiters holding a massive, solid core, such as HD 149026b. In the disk of a dwarf star, a number of Earth-sized planets can be formed within the dust ring around∼ 0.5 au, a phenomenon that potentially explains a planet system made of multiple Earth-sized planets around a dwarf star such as TRAPPIST-1.