A toner density detection apparatus has a sensor unit and a controller. The sensor unit irradiates a toner patch formed on an image carrier with light, splits the light reflected by the toner patch into first and second light components, receives the first and second light components, and outputs first and second light reception signals. The controller causes an analog/digital converter to convert, based on a reference voltage, the output first and second light reception signals into first and second digital data, and detects the density of the toner patch based on the first and second digital data. The toner density detection apparatus adjusts the toner density detection value based on a comparison between a first reference value stored in advance and the first digital data, and a comparison between a second reference value stored in advance and the second digital data.