A sheet material detecting device for identifying the material of a sheet by utilizing an impact applied to the sheet comprises an impact applying means which is supported to be rotatable and shiftable toward the sheet, and applies an impact onto the sheet, an impact receiving means which is supported to be rotatable and shiftable in a state of being in contact-with the sheet onto which the impact has been applied, arranged opposite the impact applying means with a gap permitting the passage of the sheet in-between, and receives via the sheet the impact applied by the impact applying means onto the sheet, and a signal output unit which outputs a signal in response to the impact applied onto the sheet.