An image forming apparatus includes image carriers for four fundamental colors of black, cyan, magenta, and yellow, and for an auxiliary color, and three optical scanning devices. A first optical scanning device includes two light sources for the black color and another fundamental color, a first deflector, and a first housing. A second optical scanning device includes two light sources for other two fundamental colors, a second deflector, and a second housing. A third optical scanning device includes a light source for the auxiliary color, a third deflector, a third housing, and one or more reflecting mirrors. The light source for the auxiliary color is disposed closer to the third deflector with the one or more reflecting mirrors to turn an optical path therebetween while maintaining an optical path length thereof. The optical paths for the auxiliary color and for the black color have identical light utilization efficiency.