A heat developing apparatus comprising an image exposure portion in which a heat development recording material containing a heat development photosensitive material or a photosensitive heat-sensitive recording material is exposed to light to thereby form a latent image, and a heat developing portion in which heat is applied to said latent image to thereby develop said latent image, wherein: said image exposure portion is provided as a sub scanning type structure including a planar guide plate disposed on one side with respect to a carrying path of said heat development recording material, and two drive rollers disposed on other side with respect to said carrying path and having axes substantially parallel to said carrying path and crossing a carrying direction of said heat development recording material substantially perpendicularly; and a plurality of light-emitting elements are embedded in said guide plate so as to be arranged in a main scanning direction to form a light-emitting element array.