A sheet remainder detecting device for detecting the remaining amount of sheets in a sheet stack is provided with a displacing member displaceable according to the thickness of the sheet stack; a first and a second optical sensors each having a light emitting element for emitting light toward the displacing member and a light receiving element for receiving the light emitted by the light emitting element and reflected by the displacing member; and a controller for discriminating the remaining amount of the sheets based on detection signals of the first and second optical sensors. The displacing member includes a light irradiation region to be irradiated with the lights from the light emitting elements and movable according to an amount of displacement of the displacing member.