A fuel property diagnostic device configured to diagnose properties of fuel for a vehicle includes a computer configured to diagnose the properties of the fuel for each region based on the number of vehicles with clogging. The number of vehicles with clogging is the number of vehicles in which an internal combustion engine provided with an exhaust gas control device in an exhaust passage is mounted, the exhaust gas control device includes a filter for capturing particulate matter contained in an exhaust gas, and the filter is clogged.