A reducing agent supplying device includes a reforming device, an obtaining section and a controller. The reforming device mixes fuel, which is a hydrocarbon compound, with air, and reforms the fuel by partially oxidizing the fuel with oxygen in the air. A reformed fuel is supplied into the exhaust passage as the reducing agent. The obtaining section obtains a physical quantity as a property index. The physical quantity has a correlation with property of the fuel that is supplied to the reforming device. The controller controls the reforming device according to the property index obtained by the obtaining section.