To provide a speed reducing mechanism, an image reading device and an image forming device capable of simultaneously and easily adjusting the tension of two timing belts.

A supporting member 36 rotatably supporting an intermediate pulley 32 having a first timing belt 33 and a second timing belt 34 wrapped around and capable of being rotated by a rotation center shaft 37 is rotated by energizing it with an elastic member 39 and fixed at a predetermined position with a fixing member 44. Thereby, tension can be imparted to the first timing belt 33 and the second timing belt 34, and the two timing belts 33 and 34 can be stretched by a simple structure at the same time, so that the tension of the two timing belts 33 and 34 can be adjusted simultaneously and easily.