An apparatus for forming an image of the present invention comprises an image medium which is made of a heat-sensitive material in and from whcih an image can be thermally recordded and erased. The image medium has at least two display regions having different colors in the same face and having different thermal transition temperatures between a transparent state and a scattering state. A thermal signal applying member is employed for forming an image by thermally scanning said image medium which is characterized by applying to said medium a temperature controlled so that at least two differnt colors are displayed in a one-signal period of thermal scanning. The present invention also provides a method of forming an image using the image forming apparatus.