A positively chargeable toner includes a plurality of toner particles. The toner particles each include a toner mother particle and an external additive. The toner mother particle includes a toner core containing a binder resin and a shell layer covering a surface of the toner core. The external additive includes a plurality of silica particles. The silica particles are each present on a surface of the shell layer and include a silica base having a surface treated with a surface treatment agent. The surface treatment agent includes a first treatment agent having a carboxyl group in a molecule thereof. The toner core and each of the silica particles is bonded to each other through a specific covalent bond. An amount of ring-unopened oxazoline groups included in 1 g of the positively chargeable toner as measured by gas chromatography-mass spectrometry is at least 0.10 μmol and no greater than 100 μmol.