A resin impregnating device (1) is used in a method for producing a reinforcing bar and has a chamber for holding a liquid thermoplastic resin. A plurality of guide plates (4A-4C) is arranged in the chamber along a traveling direction of a plurality of strands of reinforcing fiber material (Fb). Through holes (41) in two of the guide plates (4A, 4C) guide or spread the strands of the reinforcing fiber material Fb away from each other, and a single through hole (42) in an intermediate one of the guide plates (4B) guides or converges all the strands of the reinforcing fiber material (Fb) towards each other.