The present invention relates to a harmful emissions reduction apparatus for an engine using a reformer, a reformer for the same, and a harmful emissions reduction method for an engine using the same. The harmful emissions reduction apparatus for an engine comprises: a reformer (20) which reforms oxygen (O) and hydrocarbons (HC) included in the exhaust gas from an engine into hydrogen (H2) and carbon monoxide (CO), and increases the temperature of the exhaust gas by exothermic reaction; an exhaust gas recirculation unit (EGR) (30) which supplies the engine (10) with the exhaust gas which was reformed by the reformer (20) to increase the hydrogen (H2) content, and stabilizes the homogeneous charge compression ignition (HCCI) combustion of the engine (10); a hydrocarbon-added selective catalytic reduction unit (HC-SCR) (40) which receives the exhaust gas which was reformed by the reformer (20) to increase the hydrogen (H2) content, and transforms the nitrogen oxide (NOx) into nitrogen (N2) and water vapor (H2O) using hydrocarbons as a reducing agent in a state that enhances the reduction reaction of the nitrogen oxide (NOx); and a diesel particulate filter (DPF) (50) which receives the exhaust gas which was reformed by the reformer (20) to increase the hydrogen (H2) content, and removes particulate matter (PM) deposited thereinside through oxidation. The harmful emissions reduction apparatus for an engine of the present invention treats the exhaust gas more efficiently as post-processing apparatuses such as the selective catalytic reduction (HC-SCR) and the diesel particulate filter (DPF) are interacting in addition to the application of the HCCI combustion and the reformer.