Image forming apparatus, sheet type determination method and program in the apparatus

An image forming apparatus includes: a sheet feeding tray that stores a sheet; a sheet thickness measurement device that measures a sheet thickness of the sheet fed from the sheet feeding tray; a hardware processor that: determines a type of the sheet of which the sheet thickness is measured, a sheet type associated with each of sheet thickness ranges, and the sheet thickness measured by the sheet thickness measurement device; and adjusts a first sheet thickness range to cause the first sheet thickness range to be expandable to at least one of an upper limit value side or a lower limit value side; and an image former that forms an image on the sheet fed from the sheet feeding tray under image forming conditions corresponding to the sheet type determined by the hardware processor.