An imaging system is provided wherein images are formed with good tonal qualities in a short time immediately after development process. Concretely, the imaging system of the invention comprises a print medium, wherein microcapsules encapsulate a color-forming material, a polymerization initiator and a developer, an irradiation box, which selectively exposes the print medium, a pressure-developing roller, and first and second post heaters. After the development using the pressure-developing roller, the print medium is heated by the first post heater on the condition that the heating time Y is longer than a second and shorter than approximately 1060 exp (-0.082X) seconds, wherein X is defined as the heating temperature (degrees). Further, the print medium is heated by the second post heater on the condition that the heating time Y is shorter than a second and longer than approximately 1060 exp (-0.082X) seconds.