A diagnostic device comprising: an oxidation catalyst arranged in an exhaust system of an internal combustion engine and configured to oxidize at least hydrocarbons and nitrogen monoxide contained in an exhaust gas; a diesel particulate filter arranged in the exhaust system at a position downstream of the oxidation catalyst and configured to collect particulate matters in the exhaust gas; a selective catalytic reduction catalyst arranged in the exhaust system at a position downstream of the filter and configured to reduce and purify NOx contained in the exhaust gas with ammonia being a reducing agent; first purification rate calculation means configured to calculate a low temperature NOx purification rate when a catalyst temperature of the selective catalytic reduction catalyst is in a predetermined low temperature range based on a NOx value at an upstream side and a NOx value at a downstream side of the selective catalytic reduction catalyst, and calculate a high temperature NOx purification rate when the catalyst temperature of the selective catalytic reduction catalyst is in a predetermined high temperature range based on the NOx value at the upstream side and the NOx value at the downstream side of the selective catalytic reduction catalyst; second purification rate calculation means configured to calculate a hydrocarbon purification rate of the oxidation catalyst at the time of a forced regeneration in the filter based on at least a difference in exhaust gas heat quantity between an upstream side and a downstream side of the oxidation catalyst; and determination means configured to determine whether the oxidation catalyst is in a deteriorated state based on the calculated low temperature NOx purification rate, the calculated high temperature NOx purification rate, and the calculated hydrocarbon purification rate, wherein the determination means determines that a capability of the oxidation catalyst to generate nitrogen dioxide is in a deteriorated state when the low temperature NOx purification rate drops, the high temperature NOx purification rate does not drop, and the hydrocarbon purification rate drops. an oxidation catalyst arranged in an exhaust system of an internal combustion engine and configured to oxidize at least hydrocarbons and nitrogen monoxide contained in an exhaust gas; a diesel particulate filter arranged in the exhaust system at a position downstream of the oxidation catalyst and configured to collect particulate matters in the exhaust gas; a selective catalytic reduction catalyst arranged in the exhaust system at a position downstream of the filter and configured to reduce and purify NOx contained in the exhaust gas with ammonia being a reducing agent; first purification rate calculation means configured to calculate a low temperature NOx purification rate when a catalyst temperature of the selective catalytic reduction catalyst is in a predetermined low temperature range based on a NOx value at an upstream side and a NOx value at a downstream side of the selective catalytic reduction catalyst, and calculate a high temperature NOx purification rate when the catalyst temperature of the selective catalytic reduction catalyst is in a predetermined high temperature range based on the NOx value at the upstream side and the NOx value at the downstream side of the selective catalytic reduction catalyst; second purification rate calculation means configured to calculate a hydrocarbon purification rate of the oxidation catalyst at the time of a forced regeneration in the filter based on at least a difference in exhaust gas heat quantity between an upstream side and a downstream side of the oxidation catalyst; and determination means configured to determine whether the oxidation catalyst is in a deteriorated state based on the calculated low temperature NOx purification rate, the calculated high temperature NOx purification rate, and the calculated hydrocarbon purification rate, wherein the determination means determines that a capability of the oxidation catalyst to generate nitrogen dioxide is in a deteriorated state when the low temperature NOx purification rate drops, the high temperature NOx purification rate does not drop, and the hydrocarbon purification rate drops.