It is intended to provide a remineralization promoter for tooth enamel and a composition for an oral cavity, which can highly effectively promote the remineralization of tooth enamel and strongly inhibit dental caries. The remineralization promoter for tooth enamel and the composition for an oral cavity are characterized by containing amorphous hydroxyapatite and/or amorphized hydroxyapatite showing two peaks at 2.theta.=31 to 35.degree. in X-ray diffraction. The amorphized hydroxyapatite is obtained by reacting calcium salt with phosphate in an aqueous solution and drying the reaction solution at a temperature of 10 to 70.degree. C.