To provide an image forming device capable of efficiently forming an image by changing the time required for developing corresponding to the width of a photosensitive recording medium and matching the time required for exposing with the time required for developing.

Two detection sensors 22A and 22B are disposed in the feeding pat of a microcapsule paper and the width of the microcapsule paper is detected by a width detection means 70A of a CPU 70 based on the detected results of the sensors 22A and 22B. Corresponding to the width of the microcapsule paper, a driving motor 54 is controlled by a scanning width changing means 70B of the CPU 70 and driving motors 23 and 60 are controlled by a speed changing means 70C of the CPU 70 respectively to change the scanning width of a carriage 48, a feeding speed by feeding means 21 and 25 and a developing speed by a pressure developing means 45.