To provide a thermal transfer image forming device which permits an easy detection of the remaining amount of an ink ribbon in replacing and using an ink ribbon cassette and a method of displaying/detecting the remaining amount of the ink ribbon.

The method of displaying/detecting the remaining amount of the ink ribbon of the thermal transfer image forming device comprises printing an image by applying heat to the ink ribbon 40 in which an ink layer is formed on a translucent sheet from a thermal head 1 and transferring the ink to a paper P. A pressrun mark which can be optically detected on the ink ribbon is formed, and the pressrun mark is detected by using at least one photosensor 20 to detect the remaining amount of the ink ribbon before starting subsequent printing by applying heat to the ink ribbon from the thermal head and removing a part of the ink layer.