An image forming device includes a device body, a cartridge detachably attached to the device body so as to accommodate material which includes one of consumable material and waste material, a sensor configured to emit and receive sensing light and give a detection value corresponding to an amount of the material in the cartridge which amount is detected based upon an intensity of the sensing light received thereby, a received light changing member configured to change the intensity of the sensing light received by the sensor, and a judging unit configured to judge based upon a change of the detection value given by the sensor whether the cartridge is a new one.