Exhaust gas purifying device for internal combustion engine

An exhaust gas purifying device performs a diagnosis of operational normality or abnormality of a catalyst on the basis of a change in catalyst temperature due to an increase in the reduction substance content of exhaust gas which is caused by implementing post fuel injection near TDC of a compression stroke following main fuel injection. The timing of post fuel injection is set at a rotational angle of crankshaft between 30• and 210• after TDC of the compression stroke in order to provide a rise in temperature of the catalyst without increasing the amount of post fuel injection.