Reading device, image forming apparatus, and state detection method to detect a multi-feed state of an object using reflected light in a visible wavelength and an invisible wavelength and comparing reading value results to references associated with the invisible and visible wavelength reference values

A reading device includes an illuminator, an imaging device, a memory, and a detector. The illuminator illuminates an object to be read with light in an invisible wavelength range. The imaging device captures an image of the light reflected from the object to be read, within the invisible wavelength range. The memory holds a reference value of the object to be read. The detector detects a state of the object to be read, based on a difference between a level of a reading value of an invisible image captured within the invisible wavelength range and a level of the reference value.