An exhaust gas purifying device for an internal combustion engine includes: an electrically heated catalyst (EHC) including an insulating member; an insulation resistance detector; and a processor. The processor is configured to: acquire an insulation resistance value of the insulating member using the insulation resistance detector each time a trip of the vehicle starts; and execute diagnostic processing to diagnose the state of the EHC when the acquired insulation resistance value is equal to or less than a reference value. In the diagnostic processing, the processor is configured to determine whether or not there is an insulation abnormality of the EHC, based on an index value indicating the degree of decrease in an insulation resistance value of the insulating member of the current trip with respect to an insulation resistance value of the insulating member of one or more past trips including the last trip.