PURPOSE: To form an accurate color image in an almost instant time by incorporating a developer into microcapsules, heating the capsules with microwaves of a predetermined wavelength to collapse, and developing with the leaked developer.

CONSTITUTION: A leakage-preventing shells 5 of microcapsules 4 deposited on a base material 2 such as paper contain a developer and a photopolymn. initiator which causes photopolymn. with active radiation. The developer contains a coloring agent. As for the coloring agent, such a material is used that is activated by absorption of blue primary color while it is inactive with other green or yellow light. As for the photopolymn. initiator, for example, a compd. which can maintain inactive with blue light but becomes active with green or yellow light is used. The sheet 1 thus produced is irradiated with multicolored light while the microcapsules 4 are heated by absorption of microwaves of a predetermined wavelength. When the capsules are broken by heating to leak the developer, a given color is developed with the leaked developer to realize instant development.