To provide a toner density detection device capable of improving detecting the accuracy of toner density.

In the toner density detection device 80 detecting the toner density of a toner image carrier 61 which carries a toner image is equipped with a light source 101; a first polarized light splitting part 121 for splitting emitted light from the light source 101 to P polarized light and S polarized light, guiding the P polarized light to a toner image forming part 61a and also guiding the S polarized light to a toner image non-forming part 61b; a second polarized light splitting part 122 for splitting the light reflected from the toner image forming part 61a to P polarized light and S polarized light; a first light-receiving part 111 for receiving the P polarized light split by the second polarized light splitting part 122; a second light receiving part 112 receiving the S polarized light split by the second polarized light splitting part 122; and a third light-receiving part 102 for receiving reflected light from the toner image non-forming part 61b. The toner density is detected, based on the light quantity of the first and the second light receiving parts 111 and 112 received, and also the light quantity of the light source 101 is varied, based on the received light quantity of the third light receiving part 102.