A reducing agent supplying device includes a reaction container. The reaction container has a reaction chamber therein in which fuel of a hydrocarbon compound is mixed with air and is oxidized with oxygen in air. An equivalent ratio of fuel to air inside the reaction chamber is adjusted to be within a specified equivalent ratio range. A temperature inside the reaction chamber is adjusted to be within a specified temperature range. The specified equivalent ratio range and the specified temperature range are set such that a cool flame reaction, through which fuel inside the reaction chamber is partially oxidized with oxygen in air, is generated. The fuel partially oxidized through the cool flame reaction is used as the reducing agent.