A diagnostic apparatus that diagnoses a condition of a particulate filter provided in an exhaust passage for an internal combustion engine, comprising: an execution portion that executes a forced recovery process that oxidizes, and thereby removes particulate matter accumulated in the particulate filter by forcibly increasing a temperature of the particulate filter; an intake air amount obtaining portion that obtains an amount of intake air drawn into the internal combustion engine; a differential pressure sensor that detects a differential pressure across the particulate filter; a diagnostic portion that determines that the particulate filter is in an abnormal condition if a differential pressure gradient, that represents a ratio of an amount by which the value detected by the differential pressure sensor increases during the period where the intake air amount of the internal combustion engine is on the increase to an amount by which the intake air amount increases during the period where the intake air amount of the internal combustion engine is on the increase, is equal to or smaller than a predetermined value; characterized by comprising: a determination portion that determines whether or not a condition where a continuous recovery, in which particulate matter accumulated in the particulate filter is oxidized due to an increase in an exhaust gas temperature, occurs is satisfied in the period where the forced recovery process by the execution portion is unexecuted; a calculation portion that calculates, if there is a period where the intake air amount of the internal combustion engine is on an increase when the forced recovery process by the execution portion is unexecuted and the determination portion determines that the condition where the continuous recovery occurs is not satisfied, the differential pressure gradient. an execution portion that executes a forced recovery process that oxidizes, and thereby removes particulate matter accumulated in the particulate filter by forcibly increasing a temperature of the particulate filter; an intake air amount obtaining portion that obtains an amount of intake air drawn into the internal combustion engine; a differential pressure sensor that detects a differential pressure across the particulate filter; a diagnostic portion that determines that the particulate filter is in an abnormal condition if a differential pressure gradient, that represents a ratio of an amount by which the value detected by the differential pressure sensor increases during the period where the intake air amount of the internal combustion engine is on the increase to an amount by which the intake air amount increases during the period where the intake air amount of the internal combustion engine is on the increase, is equal to or smaller than a predetermined value; characterized by comprising: a determination portion that determines whether or not a condition where a continuous recovery, in which particulate matter accumulated in the particulate filter is oxidized due to an increase in an exhaust gas temperature, occurs is satisfied in the period where the forced recovery process by the execution portion is unexecuted; a calculation portion that calculates, if there is a period where the intake air amount of the internal combustion engine is on an increase when the forced recovery process by the execution portion is unexecuted and the determination portion determines that the condition where the continuous recovery occurs is not satisfied, the differential pressure gradient. a determination portion that determines whether or not a condition where a continuous recovery, in which particulate matter accumulated in the particulate filter is oxidized due to an increase in an exhaust gas temperature, occurs is satisfied in the period where the forced recovery process by the execution portion is unexecuted; a calculation portion that calculates, if there is a period where the intake air amount of the internal combustion engine is on an increase when the forced recovery process by the execution portion is unexecuted and the determination portion determines that the condition where the continuous recovery occurs is not satisfied, the differential pressure gradient.