An exhaust gas purification device includes: a catalyst downstream from an enlarged diameter flow path in an exhaust gas flow path; a diffusing member upstream from the enlarged diameter flow path in the exhaust gas flow path and that causes exhaust gas that has flowed in from upstream to flow out so that said exhaust gas diffuses toward the enlarged diameter flow path; a supply device that supplies a reducing agent upstream from the diffusing member in the exhaust gas flow path; and a guiding member upstream from the diffusing member either at a flow mixing position in the exhaust gas flow path where the reducing agent and the exhaust gas mix or at a position downstream from the flow mixing position, and that splits the flow of the reducing agent that has flowed in from upstream into a plurality of flows and guides said flows toward the diffusing member.