A thickness detection mechanism arranged in a scanner or printer for measuring the thickness of mediums fed through the scanner or printer, which comprising: a conveyor section; at least a idle roller set arranged above the conveyor section, which includes at least a shaft and an idle roller pivotally arranged on the shaft; at least an elastic portion arranged above the shaft; and at least a pressure sensor secured on the elastic portion; wherein the thickness detection mechanism measures the thickness of the medium by measuring the pressure applied on the pressure sensor by the idle roller set as the idle roller set being pushed upward, and thus avoids the medium too thin or too thick to enter the scanner or the printer.