A printing medium detecting device capable of accurately detecting an amount of printing media in a paper cassette, an image forming apparatus including the printing medium detecting device, and a method to detect a printing medium. The printing medium detecting device of the image forming apparatus includes a light source to emit a light beam to one side of a stack of printing media housed in a paper cassette, a scanning unit to scan the light beam reflected from the stack and generate a signal based on the light beam, and a computing unit to compute information related to the stack based on the signal, wherein the information includes a number of printing media sheets in the stack.