To provide a toner detecting device which can precisely detect the state of toner in a toner storage part without any rise in cost by reducing the sticking of toner to a transparent window part with simple constitution.

A detection part 493 is so constituted as to detect the state of the toner in the toner storage part through transparent window parts 4933K and 4934K. In concrete, the detection part 493K is constituted by using a light emission part 493K which irradiates the inside of the toner storage part with light and a light reception part 4932K which receives the light and provided with the transparent window parts 4933K and 4934K on the path of the light so that toner is prevented from entering the light emission part 4931K, etc. The transparent window parts are formed by selecting a material where toner is hardly stuck according to the difference in work function from the toner and the tilt of an electron discharge amount to light energy, so the toner detecting device can precisely detect the state of the toner in the toner storage part.