A PM emission amount estimation device for a diesel engine provided with a Diesel Particulate Filter DPF collecting a Particulate Matter PM that is emitted from an exhaust gas passage of the engine, the PM emission amount estimation device comprising and configured so that: a PM emission amount estimation base-map with which a base PM emission amount is computed in response to an operating state of the engine; a PM emission amount estimation correction-device that determines a correction factor for correcting the base PM emission amount computed by the PM emission amount estimation base-map in response to a transient state of the engine; a transient state judgment device that judges that the engine is operated in the transient state based on a change of an air excess ratio regarding the engine; a PM emission amount computation device that corrects the computed base PM emission amount by using the correction factor computed with the PM emission amount estimation correction-device only in a case where the transient state judgment device judges that the engine is in a transient operation condition, whereas the base PM emission amount is outputted in a case where the transient state judgment device judges that the engine is in a steady operation condition; and a PM emission accumulation amount device that integrates the PM emission amount which is computed in the PM emission amount computation device, characterized in that the PM emission amount estimation device further comprises and is further configured so that: said transient state judgment device that judges that the engine is operated in the transient state based on a change of an air excess ratio regarding the engine; wherein the transient state judgment device judges whether the engine is operated in a transient operation condition or a steady operation condition, in a manner that: a current value of the air excess ratio is subtracted by a former value of the air excess ratio; the subtracted value is divided by a measurement time interval to obtain a calculated value; and the judgment whether the engine is operated in a transient operation condition or a steady operation condition is judged by comparing the calculated value with a threshold value, and wherein the threshold value is set so that a PM emission integrated amount which is computed in the PM emission accumulation amount device agrees with an actual PM emission integrated amount. a PM emission amount estimation base-map with which a base PM emission amount is computed in response to an operating state of the engine; a PM emission amount estimation correction-device that determines a correction factor for correcting the base PM emission amount computed by the PM emission amount estimation base-map in response to a transient state of the engine; a transient state judgment device that judges that the engine is operated in the transient state based on a change of an air excess ratio regarding the engine; a PM emission amount computation device that corrects the computed base PM emission amount by using the correction factor computed with the PM emission amount estimation correction-device only in a case where the transient state judgment device judges that the engine is in a transient operation condition, whereas the base PM emission amount is outputted in a case where the transient state judgment device judges that the engine is in a steady operation condition; and a PM emission accumulation amount device that integrates the PM emission amount which is computed in the PM emission amount computation device, a current value of the air excess ratio is subtracted by a former value of the air excess ratio; the subtracted value is divided by a measurement time interval to obtain a calculated value; and the judgment whether the engine is operated in a transient operation condition or a steady operation condition is judged by comparing the calculated value with a threshold value,