An image forming method including preparing a thermal transfer sheet having a dye layer on a substrate, the dye layer containing a sublimable dye, preparing a decorative seal having a seal base material on a release sheet, the seal base material having a layered structure of an adhesive layer and a film layered in this order from the release sheet side, the film containing a resin having a dyeing affinity to the sublimable dye contained in the dye layer, and the film having an elongation percentage of not less than 150% and not more than 400% when determined by a test method in conformity with JIS-K-7127:1999, and thermally transferring the sublimable dye in the dye layer onto the seal base material by sublimation thermal transfer process, thereby forming a thermal transfer image on the seal base material.