In an optical printing system for printing an image on a recording sheet, adopted as the recording sheet is a sheet coated with microcapsules on one surface thereon. Each microcapsule envelopes a first component which changes color when reacts with a second component. The second component may be coated on the recording sheet or may be coated on a separate developing sheet. The microcapsule further envelopes a third component which varies the mechanical strength of microcapsule when light is projected thereto. An image is formed on the recording sheet by selectively exposing a light thereto in accordance with the image to be printed thereon, and the microcapsules on the recording sheet whose mechanical strength is low are then ruptured by rupturing means. Thereby, the visible image is developed. The exposing is carried out by repeating a predetermined-period exposure at a predetermined interval against the same portion of the recording sheet. The printing operation of the printing system is executed by control means in accordance with a predetermined sequence.