According to one embodiment, an image forming apparatus, includes a transfer belt onto which toner images are transferred during an image forming operation, and a sensor that emits light towards the transfer belt and detect an amount of light reflected by the transfer belt. A processor adjusts the amount of light emitted towards the transfer belt based on the detected amount of the reflected light, and stores, in a memory, light level information indicating an output level for the amount of light emitted towards the transfer belt by the sensor. The processor provides an instruction for outputting warning information when the light level information indicates the output level exceeds a threshold value.