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 contained in exhaust gas of an internal combustion engine. The reducing agent supplying device supplies a reducing agent into the exhaust passage at a position upstream of the reducing catalyst. The reducing agent supplying device includes a reforming portion and a reformation suppressing portion. The reforming portion reforms the reducing agent by partially oxidizing the reducing agent. When a temperature of the reducing agent is higher than a first specified temperature that is equal to or higher than an activation temperature of the reducing catalyst, the reformation suppressing portion (i) suppresses the degree of reformation of the reducing agent in comparison with when a temperature of the reducing agent is lower than the first specified temperature, or (ii) stops the reformation of the reducing agent.