A diagnostic device includes: a DOC for oxidizing hydrocarbon (HC) contained in an exhaust gas; a first sensor for detecting an exhaust gas temperature at a DOC inlet; a second sensor for detecting an exhaust gas temperature at a DOC outlet; a third sensor for detecting ambient temperature; an HC heat generation rate calculation unit which calculates, based on detection values of the first and second sensors, a difference in an exhaust gas heat quantity between upstream and downstream sides of the DOC, calculates, based on the detection values of the three sensors, a heat loss quantity released to outside air from the DOC, and adds the heat loss quantity and the exhaust gas heat quantity difference to calculate a heat generation quantity of HC in the DOC; and a DOC deterioration determination unit which determines, based on the calculated HC heat generation quantity, deterioration of the DOC.