To prevent degradation of purification efficiency of exhaust gas by suppressing variation of the way reducing agent spreads into the exhaust gas even when an exhaust gas flow velocity varies, in a reducing agent supply device 1.

According to this reducing agent supply device 1, an addition valve 7 is attached to an exhaust pipe 6 and directly injects a reducing agent discharged from a supply pump 5 into the exhaust pipe 6, and an ECU 8 controls discharge pressure of the supply pump 5 according to an exhaust gas flow velocity. Thereby, when the exhaust gas flow velocity is large, reducing agent spray can be radially expanded against the flow of the exhaust gas by increasing penetration force of the reducing agent spray by increasing the pressure of the injected reducing agent. On the contrary, when the exhaust gas flow velocity is small, rapid radial expansion of the reducing agent spray can be suppressed by reducing the penetration force of the reducing agent spray by reducing the pressure of the injected reducing agent. As a result, degradation of purification efficiency of the exhaust gas can be prevented by suppressing variation of the way the reducing agent spreads into the exhaust gas is suppressed even if the exhaust gas flow velocity changes.