A reducing agent supplying device is for a fuel combustion system that includes a NOx purifying device with a reducing catalyst arranged in an exhaust passage to purify NOx. The reducing agent supplying device supplies a reducing agent into the exhaust passage upstream of the reducing catalyst. The reducing agent supplying device includes an air pipe, a vaporizing container, an injector, a heating member and a mixing plate. The air pipe defines an air passage therein. The vaporizing chamber is connected to the air pipe. The vaporizing container defines a vaporizing chamber therein that branches off from the air passage. The injector injects liquid hydrocarbon compound as a reducing agent. The heating member is disposed in the vaporizing chamber. The heating member heats and vaporizes the reducing agent. The mixing plate is connected to the heating member. The mixing plate extends from the vaporizing chamber into the air passage.