A reducing agent supplying device is for a fuel combustion system that includes a NOx purifying device (15) with a reducing catalyst arranged in an exhaust passage (10ex) to purify NOx contained in exhaust gas of an internal combustion engine (10). The reducing agent supplying device supplies a reducing agent into the exhaust passage (10ex) at a position upstream of the NOx purifying device (15). The reducing agent supplying device includes an air pipe (23), a vaporizing container (25), an injector (70), a heating member (30, 40) and a mixing plate (50). The air pipe defines an air passage (23b) therein. The vaporizing container (25) defines a vaporizing chamber (25a) therein that branches off from the air passage (23b). The vaporizing chamber (25a) is connected to the air pipe (23). The injector (70) injects liquid hydrocarbon compound as a reducing agent. The heating member (30, 40) is disposed in the vaporizing chamber (25a). The heating member (30, 40) heats and vaporizes the reducing agent injected from the injector (70). The mixing plate (50) is connected to the heating member (30, 40). The mixing plate extends from the vaporizing chamber (25a) into the air passage (23b).