A toner for developing an electrostatic image has toner particles containing a binder resin and a colorant. The toner particle has at least a core composed of a low-temperature softening substance, an inner layer enclosing the core, and an outer layer enclosing the core and the inner layer. The core, the inner layer, and the outer layer are constituted respectively of a material which is discriminable by staining with triruthenium tetroxide and triosmium tetroxide.