A signal output apparatus comprises an impact applying unit applying an impact to a sheet from the outside thereof, and a detection unit outputting a signal by the impact. An apparatus for determining the type of sheet comprises an impact applying unit applying an impact to a sheet from the outside thereof, and a detection unit outputting a signal by the impact, wherein the type of the sheet is determined based on the signal from the detection unit. An image forming apparatus comprises an impact applying unit applying an impact to a sheet from the outside thereof, and a detection unit outputting a signal by the impact. A method for determining the type of sheet comprises the steps of applying an impact to a sheet from the outside thereof, outputting a signal from a detection unit by the applying step, and determining the type of sheet based on the signal. An apparatus carries out the method for determining the type of sheet. An information output apparatus used in an image forming apparatus comprises an impact applying unit applying an impact to a target from the outside thereof, and a detection unit outputting information by the impact.